Undefined control sequence \reset@newl@bel

Question

\relax 
\reset@newl@bel
\select@language{english}
\@writefile{toc}{\select@language{english}}
\@writefile{lof}{\select@language{english}}
\@writefile{lot}{\select@language{english}}
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\@writefile{toc}{\contentsline {section}{\numberline {2}REQUIREMENTS}{2}}
\@writefile{toc}{\contentsline {subsection}{\numberline {2.1}User Authentication}{2}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.1.1}Create Account}{2}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.1.2}Sign in}{2}}
\@writefile{toc}{\contentsline {subsection}{\numberline {2.2}Preference}{2}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.2.1}Input initial preference}{2}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.2.2}Analysis with collaborative filtering algorithm}{2}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.2.3}Recommendation of music based on genre}{2}}
\@writefile{toc}{\contentsline {subsection}{\numberline {2.3}My Page}{2}}
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\@writefile{toc}{\contentsline {paragraph}{\numberline {2.3.1.1}Profile picture}{2}}
\@writefile{toc}{\contentsline {paragraph}{\numberline {2.3.1.2}Profile music}{2}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.3.2}Posting with music}{2}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.3.3}Edit / delete post}{2}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.3.4}Playlist}{3}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.3.5}My box}{3}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.3.6}Hash tag}{3}}
\@writefile{toc}{\contentsline {paragraph}{\numberline {2.3.6.1}Search}{3}}
\@writefile{toc}{\contentsline {paragraph}{\numberline {2.3.6.2}Following}{3}}
\@writefile{toc}{\contentsline {subsection}{\numberline {2.4}Music Feed}{3}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.4.1}Note Button}{3}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.4.2}Save to My Box}{3}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.4.3}Comment}{3}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {2.4.4}Report}{3}}
\@writefile{toc}{\contentsline {subsection}{\numberline {2.5}Friends}{3}}
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\@writefile{toc}{\contentsline {subsection}{\numberline {2.9}Question Board}{3}}
\@writefile{toc}{\contentsline {subsection}{\numberline {2.10}Frequently uploaded Music TOP10}{4}}
\@writefile{toc}{\contentsline {subsection}{\numberline {2.11}Voting function}{4}}
\@writefile{toc}{\contentsline {section}{\numberline {3}Development}{4}}
\@writefile{toc}{\contentsline {subsection}{\numberline {3.1}Choice of software development platform}{4}}
\@writefile{toc}{\contentsline {subsection}{\numberline {3.2}Software in use}{4}}
\@writefile{toc}{\contentsline {section}{\numberline {4}Specifications}{4}}
\@writefile{toc}{\contentsline {subsection}{\numberline {4.1}User Authentication}{4}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {4.1.1}Create Account}{4}}
\@writefile{lof}{\contentsline {figure}{\numberline {1}{\ignorespaces Sample register page}}{4}}
\newlabel{fig1}{{1}{4}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {4.1.2}Sign in}{5}}
\@writefile{lof}{\contentsline {figure}{\numberline {2}{\ignorespaces Sample login page}}{5}}
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\@writefile{toc}{\contentsline {subsection}{\numberline {4.2}Preference}{5}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {4.2.1}Input initial preference}{5}}
\@writefile{lof}{\contentsline {figure}{\numberline {3}{\ignorespaces Collecting user preference data with Google Forms}}{5}}
\newlabel{fig3}{{3}{5}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {4.2.2}Analysis with collaborative filtering algorithm}{5}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {4.2.3}Recommendation of music based on genre}{8}}
\@writefile{toc}{\contentsline {subsection}{\numberline {4.3}My Page}{8}}
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\@writefile{toc}{\contentsline {paragraph}{\numberline {4.3.1.2}Profile music}{9}}
\@writefile{toc}{\contentsline {paragraph}{\numberline {4.3.1.3}Posting with music}{9}}
\@writefile{lof}{\contentsline {figure}{\numberline {6}{\ignorespaces An example post which includes a song}}{9}}
\newlabel{fig6}{{6}{9}}
\@writefile{toc}{\contentsline {subsection}{\numberline {4.4}Friends}{9}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {4.4.1}Search}{9}}
\@writefile{toc}{\contentsline {subsubsection}{\numberline {4.4.2}Recommendation of friends}{9}}
\@writefile{toc}{\contentsline {subsection}{\numberline {4.5}Recommendation Playlist based on mood}{9}}
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\newlabel{overflow}{{7}{9}}
\@writefile{toc}{\contentsline {subsection}{\numberline {4.6}Chatting}{9}}
\@writefile{lof}{\contentsline {figure}{\numberline {8}{\ignorespaces Public Chatroom }}{10}}
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\newlabel{overflow}{{9}{10}}
\@writefile{toc}{\contentsline {subsection}{\numberline {4.7}Message Box}{10}}
\@writefile{lof}{\contentsline {figure}{\numberline {10}{\ignorespaces The location of message box }}{10}}
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\@writefile{lof}{\contentsline {figure}{\numberline {11}{\ignorespaces The writing box of Message Box}}{10}}
\newlabel{overflow}{{11}{10}}
\@writefile{toc}{\contentsline {subsection}{\numberline {4.8}Question Board}{10}}
\@writefile{lof}{\contentsline {figure}{\numberline {12}{\ignorespaces The Question Board }}{11}}
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\@writefile{toc}{\contentsline {subsection}{\numberline {4.9}Music Top 10}{11}}
\@writefile{lof}{\contentsline {figure}{\numberline {14}{\ignorespaces The current Top 10 Playlist }}{11}}
\newlabel{overflow}{{14}{11}}
\@writefile{toc}{\contentsline {subsection}{\numberline {4.10}Voting function}{11}}
\@writefile{lof}{\contentsline {figure}{\numberline {15}{\ignorespaces The example of voting }}{11}}
\newlabel{overflow}{{15}{11}}
\bibcite{IEEEhowto:kopka}{1}
\@writefile{toc}{\contentsline {section}{Appendix\nobreakspace  A: Proof of the First Zonklar Equation}{12}}
\@writefile{toc}{\contentsline {section}{Appendix\nobreakspace  B}{12}}
\@writefile{toc}{\contentsline {section}{References}{12}}
\@writefile{toc}{\contentsline {section}{Biographies}{12}}
\@writefile{toc}{\contentsline {subsection}{Michael Shell}{12}}
\@writefile{toc}{\contentsline {subsection}{John Doe}{12}}
\@writefile{toc}{\contentsline {subsection}{Jane Doe}{12}}

Answer 1

Very likely you are using package acronym and have updated it recently. Older version have used \reset@newl@bel in the .aux files. Recently this was renamed to \AC@reset@newl@bel. Older .aux files are causing now the undefined command error. Either you can ignore the error or delete the .aux files. The error should be gone, when the .aux files are rewritten with the new version of acronym.