The argument of every \cite
instruction -- the "citation key" -- in the body of the document must correspond exactly to the mandatory argument of a \bibitem
instruction in the thebibliography
environment. (\bibitem
can also have an optional argument. If the \bibitem
statements don't have optional arguments, LaTeX will form numeric-style citation call-outs.) And, be sure to run LaTeX twice.
Here's an MWE (minimum working example) involving two \cite
instructions and two \bibitem
statements.

\documentclass{article}
\begin{document}
\cite{aa}, \cite{bb}
\begin{thebibliography}{2}
\bibitem{aa} Anna Anderegg, 2019, Thoughts.
\bibitem{bb} Brenda Bradshaw, 2018, Worries.
\end{thebibliography}
\end{document}
\bibitem
s forthebibliography
manually, there is no need for BibTeX and.bib
files. Usually\bibitem
has one argument that can be used in\cite
, for example:\bibitem{sigfridsson} E. Sigfridsson and U. Ryde: Some Chemistry Title, J. Chem. 1998
will be cited as\cite{sigfridsson}
. Can you add a short example document that shows what you are doing to the question (an MWE: tex.meta.stackexchange.com/q/228/35864). Note that you have to compile your document at least twice to see the correct reference in the citations.\cite
instruction must correspond to the mandatory argument of a\bibitem
instruction.