Getting errors using Acro for abbreviations and acronyms: Undefined Control Sequence in acro.sty

Question

I've been trying to use acro for abbreviating terms in a report. It mostly works but each compile requires intervention to complete and does something to my first page, leaves an =1 and a page break. Any help would be greatly appreciated.

\documentclass[A4, 12pt]{article}

% for abbreviations and acronyms
\usepackage[first-style=long]{acro}

\DeclareAcronym{ma}{
  short = MA ,
  long  = Multiple Access ,
}

\DeclareAcronym{noma}{
  short = NOMA ,
  long  = Non-Orthongonal Multiple Access ,
}

% title info
\title{Beamforming for Millimeter-wave \acs{noma}}
\author{Cian Dowd}
\date{21/01/2019}

\begin{document}
\pagenumbering{roman}
\maketitle

% ABSTRACT
\begin{abstract}
    In this paper\ldots
\end{abstract}

% TABLE OF CONTENTS
\tableofcontents

% LIST OF ACRONYMS
\printacronyms

% INTRODUCTION
\pagenumbering{arabic}
\setcounter{page}{1}
\setcounter{section}{0}
\section{Introduction}
\ac{noma}\ldots


% LITERATURE REVIEW
\setcounter{section}{1}
\section{Background/ Literature Review}

% Background on Current MA
\subsection{Existing \acf{ma}}

\end{document}

This yields the following document; errors shown below it.

page break

I get these messages, ending with the errors:

(/usr/local/texlive/2016basic/texmf-dist/tex/latex/acro/acro.sty
(/usr/local/texlive/2016basic/texmf-dist/tex/latex/l3kernel/expl3.sty
(/usr/local/texlive/2016basic/texmf-dist/tex/latex/l3kernel/expl3-code.tex)
(/usr/local/texlive/2016basic/texmf-dist/tex/latex/l3kernel/l3pdfmode.def))
(/usr/local/texlive/2016basic/texmf-dist/tex/latex/l3packages/xparse/xparse.sty
)
(/usr/local/texlive/2016basic/texmf-dist/tex/latex/l3packages/l3keys2e/l3keys2e
.sty)
(/usr/local/texlive/2016basic/texmf-dist/tex/latex/l3packages/xtemplate/xtempla
te.sty)
(/usr/local/texlive/2016basic/texmf-dist/tex/latex/etoolbox/etoolbox.sty)
(/usr/local/texlive/2016basic/texmf-dist/tex/latex/oberdiek/zref-abspage.sty
(/usr/local/texlive/2016basic/texmf-dist/tex/latex/oberdiek/zref-base.sty
(/usr/local/texlive/2016basic/texmf-dist/tex/generic/oberdiek/ltxcmds.sty)
(/usr/local/texlive/2016basic/texmf-dist/tex/generic/oberdiek/infwarerr.sty)
(/usr/local/texlive/2016basic/texmf-dist/tex/generic/oberdiek/kvsetkeys.sty
(/usr/local/texlive/2016basic/texmf-dist/tex/generic/oberdiek/etexcmds.sty
(/usr/local/texlive/2016basic/texmf-dist/tex/generic/oberdiek/ifluatex.sty)))
(/usr/local/texlive/2016basic/texmf-dist/tex/generic/oberdiek/kvdefinekeys.sty)
 (/usr/local/texlive/2016basic/texmf-dist/tex/generic/oberdiek/pdftexcmds.sty
(/usr/local/texlive/2016basic/texmf-dist/tex/generic/oberdiek/ifpdf.sty))
(/usr/local/texlive/2016basic/texmf-dist/tex/latex/oberdiek/auxhook.sty))
(/usr/local/texlive/2016basic/texmf-dist/tex/generic/oberdiek/atbegshi.sty))
Runaway argument?
{\bool_if:NT \l__acro_sort_bool {\cs_new_protected:Npn \acro_sort_prop:NN \ETC.
! Forbidden control sequence found while scanning use of \g@addto@macro.
<inserted text> 
                \par 
l.2965                         = \c_minus_one

This corresponds to line 2965 of acro.sty, though I am not sure why nor do I know how to fix this issue. Relevant snippet of acro.sty below:

        \seq_sort:Nn \l__acro_tmpa_seq
          {
            \int_compare:nTF
              {
                \acro_strcmp:nn
                  { \str_fold_case:n {##1} }
                  { \str_fold_case:n {##2} }
                    = \c_minus_one
              }
              { \sort_return_same: }
              { \sort_return_swapped: }
          }

Answer 1

The constant \c_minus_one was removed from LaTeX3 in the end-of-year deprecation round for 2018. acro.sty is still using it, though, and that causes issues with new versions of the LaTeX3 kernel. See https://bitbucket.org/cgnieder/acro/issues/116/acro-errors-due-to-update-c_minus_one.

You could manually edit acro.sty and change the \c_minus_one to -1. Keep in mind, however, that usually edits like this to distributed files are strongly discouraged. In this case the edit could be justified because the file is unusable without it and an updated version of acro.sty that would make the fix unnecessary would overwrite your fixed version.

You could also reinstate the command (similar to https://github.com/latex3/latex3/commit/c96faed90a5a63ba84695190560bdacac1dfdacc#diff-645783629830bdaaeede08f6ea12cb15) before acro is loaded.

\usepackage{expl3}
\ExplSyntaxOn
\tex_let:D \c_minus_one \scan_stop:
\int_const:Nn \c_minus_one {-1}
\ExplSyntaxOff

% for abbreviations and acronyms
\usepackage[first-style=long]{acro}

edit egreg kindly suggested a streamlined version of my original suggestion.

I guess the LaTeX3 team don't really want to see that either, so until acro gets fixed by the maintainer you are stuck with several bad options.