Consider the following minimal document:
\documentclass{article}
\usepackage{fontspec}
\newcommand{\сlient_name}{Имя}
\begin{document}
\client_name
\end{document}
Running XeLaTeX over it reveals
! Missing $ inserted.
<inserted text>
$
l.4 \newcommand{\сlient_name}{Имя}
?
! LaTeX Error: Missing \begin{document}.
See the LaTeX manual or LaTeX Companion for explanation.
Type H <return> for immediate help.
...
l.4 \newcommand{\сlient_name}{Имя}
?
! Missing number, treated as zero.
<to be read again>
n
l.4 \newcommand{\сlient_name}{Имя}
?
! You already have nine parameters.
\reserved@a ...xpandafter \сlient \reserved@b #10
n{
l.4 \newcommand{\сlient_name}{Имя}
?
! Missing $ inserted.
<inserted text>
$
l.5
?
No file mohnstrudel.aux.
! Undefined control sequence.
l.9 \client
_name
?
! Missing $ inserted.
<inserted text>
$
l.9 \client_
name
?
! Missing $ inserted.
<inserted text>
$
l.10
?
Note the first error is exactly when \newcommand is being attempted. What happens later is completely unpredictable.
Fix the error, which is having _ in the command name: this is not allowed, see Command/macro name cannot include numbers and symbols
./technical_specification.tex:236: Package microtype Error: Font expansion does not work with xetex.– mohnstrudel Apr 26 '17 at 17:17\clientname. – egreg Apr 26 '17 at 17:40