This code compils on texlive 2014, 2016 and with overleaf. With Texlive2017 impossible :! Missing number, treated as zero. and ! Illegal unit of measure (pt inserted).
\documentclass[10pt, a4paper]{article}
\usepackage{amsmath,amssymb,amsthm}
\usepackage[top=2.5cm, bottom=2cm, left=2cm , right=2cm]{geometry} %permet de définir les marges
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{tikz}
\usepackage[europeanresistors,americaninductors]{circuitikz}
\usetikzlibrary{babel}
\makeatletter
\pgfdeclareshape{op amp eur}
{
\anchor{center}{\pgfpointorigin}
\savedanchor\northwest{%
\pgf@y= \pgfkeysvalueof{/tikz/circuitikz/bipoles/length}
%\pgf@y=\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/height}
\pgf@y=.4\pgf@y
\pgf@x= \pgfkeysvalueof{/tikz/circuitikz/bipoles/length}
%\pgf@x=\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/height}
\pgf@x=-\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/width}\pgf@x
\pgf@x=.5\pgf@x
}
\anchor{south}{
\northwest
\pgf@y=-\pgf@y
}
\anchor{north}{
\northwest
}
\savedanchor\left{%
\pgf@y=0pt
}
\savedanchor\inOne{%
\pgf@y= \pgfkeysvalueof{/tikz/circuitikz/bipoles/length}
%\pgf@y=\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/height}
\pgf@y=\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/height}\pgf@y
\pgf@y=.4\pgf@y
\pgf@y=\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/input height}\pgf@y
\pgf@x= \pgfkeysvalueof{/tikz/circuitikz/bipoles/length}
%\pgf@x=\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/height}
\pgf@x=-\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/width}\pgf@x
\pgf@x=.5\pgf@x
}
\anchor{-}{
\inOne
}
\anchor{+}{
\inOne
\pgf@y=-\pgf@y
}
\savedanchor\up{%
\pgf@y= \pgfkeysvalueof{/tikz/circuitikz/bipoles/length}
%\pgf@y=\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/height}
\pgf@y=\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/height}\pgf@y
\pgf@y=.4\pgf@y
\pgf@x= \pgfkeysvalueof{/tikz/circuitikz/bipoles/length}
\pgf@x=-\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/width}\pgf@x
\pgf@x=.5\pgf@x
\pgf@circ@res@up = \pgf@y
\pgf@circ@res@right = -\pgf@x
\pgf@circ@res@left = \pgf@x
\pgfpointlineattime{
\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/up pos}}{
\pgfpoint{
\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/port width}\pgf@circ@res@left}
{\pgf@circ@res@up}}
{\pgfpoint{.7\pgf@circ@res@right}{0pt}}
}
\anchor{up}{
\up
}
\anchor{down}{
\up
\pgf@y=-\pgf@y
}
\anchor{out}{
\northwest
\pgf@y=0pt
\pgf@x=-\pgf@x
}
\anchor{west}{
\left
}
\anchor{east}{
\left
\pgf@x=-\pgf@x
}
\backgroundpath{
\pgfsetcolor{\pgfkeysvalueof{/tikz/circuitikz/color}}
\northwest
\pgf@circ@res@up = \pgf@y
\pgf@circ@res@down = -\pgf@y
\pgf@circ@res@right = -\pgf@x
\pgf@circ@res@left = \pgf@x
\pgfpathmoveto{\pgfpoint
{\pgf@circ@res@left}
{\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/input height}\pgf@circ@res@up}}
\pgfpathlineto{\pgfpoint
{\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/port width}\pgf@circ@res@left}
{\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/input height}\pgf@circ@res@up}}
\pgftext[left, at=\pgfpoint{\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/port width}\pgf@circ@res@left}{\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/input height}\pgf@circ@res@up}]{\scriptsize\ $-$}
\pgfpathmoveto{\pgfpoint
{\pgf@circ@res@left}
{\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/input height}\pgf@circ@res@down}}
\pgfpathlineto{\pgfpoint
{\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/port width}\pgf@circ@res@left}
{\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/input height}\pgf@circ@res@down}}
\pgftext[left, at=\pgfpoint{\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/port width}\pgf@circ@res@left}{\pgfkeysvalueof{/tikz/circuitikz/tripoles/op amp/input height}\pgf@circ@res@down}]{\scriptsize\ $+$}
\pgfpathmoveto{\pgfpoint{\pgf@circ@res@right}{0pt}}
\pgfpathlineto{\pgfpoint{.7\pgf@circ@res@right}{0pt}}
\pgfusepath{draw}
\pgfscope
\pgfsetlinewidth{2\pgflinewidth}
\pgftransformxshift{.7\pgf@circ@res@left}
\pgf@circ@res@step=\pgf@circ@res@right
\advance\pgf@circ@res@step by -\pgf@circ@res@left
\pgf@circ@res@step=.7\pgf@circ@res@step
\pgfpathmoveto{\pgfpoint{\pgf@circ@res@step}{0pt}}
\pgfpathlineto{\pgfpoint{\pgf@circ@res@step}{\pgf@circ@res@up}}
\pgfpathlineto{\pgfpoint{0pt}{\pgf@circ@res@up}}
\pgfpathlineto{\pgfpoint{0pt}{\pgf@circ@res@down}}
\pgfpathlineto{\pgfpoint{\pgf@circ@res@step}{\pgf@circ@res@down}}
\pgfpathlineto{\pgfpoint{\pgf@circ@res@step}{0pt}}
\pgftext[left, at=\pgfpoint{0.52\pgf@circ@res@step}{-0.6\pgf@circ@res@down}]{$\rhd \infty$}
\pgfusepath{draw}
\endpgfscope
}
}
\makeatother
\begin{document}
\begin{tikzpicture}
\draw (3,3) node[op amp eur](opamp){};
\end{tikzpicture}
\end{document}
Where does the problem come from?
\pgfversionin the body of your documents in different versions. It will print out the version of TikZ\pgfkeys{/tikz/circuitikz/bipoles/length/.initial=1.4cm}. But you should ask the author if this is intended.