[Tex/LaTex] \mathbb not working in \underset

Question

I'm trying to put \mathbb{R} in an \underset of \sup in the following way:

\underset{x \in \mathbb{R}}{\sup}

However this sequence does not render as usual. I use following libraries:

\usepackage{amssymb,amsmath, amsthm}
\usepackage{enumitem}
\usepackage{microtype}
\usepackage{idxlayout}
\usepackage{amssymb}
\usepackage{lmodern}
\usepackage{amsmath}
\usepackage{float}
\usepackage{amsthm}
\usepackage{flexisym}
\usepackage{graphicx}
\usepackage{mathtools}

I'm new to LaTeX and not sure how to approach this problem. Please help.

Full MWE to reproduce this:

\documentclass[oneside,12pt,a4paper]{memoir}
\renewcommand{\baselinestretch}{1.15}
\let\memoldbibsection\bibsection
\let\bibsection\relax
\usepackage[nobysame]{amsrefs}
\let\bibsection\memoldbibsection 
\usepackage[right=2.7cm,left=3.5cm, top=2.7cm, bottom=2.5cm,includehead]{geometry}
\usepackage{amssymb,amsmath, amsthm}
\usepackage{enumitem}
\usepackage{microtype}
\usepackage{idxlayout}
\usepackage{amssymb}
\usepackage{lmodern}
\usepackage{amsmath}
\usepackage{float}
\usepackage{amsthm}
\usepackage{flexisym}
\usepackage{graphicx}
\usepackage{mathtools}

\makeindex
\makeindex[sym]


\setcounter{secnumdepth}{2}


\makeatletter

\begin{document}
$\underset{x \in \mathbb{R}}{\sup}$
\end{document}

Answer 1

If you delete \usepackage{flexisym} it works well.

In my experience, to put so many packages, often it create conflict or bad compilation. It is important, also, the order of the list the various packages.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amssymb,amsthm}
\usepackage{enumitem}
\usepackage{microtype}
\usepackage{idxlayout}
\usepackage{lmodern}
\usepackage{float}
\usepackage{amsthm}
%\usepackage{flexisym}
\usepackage{graphicx}
\usepackage{mathtools}

\begin{document}    

$\underset{x \in \mathbb{R}}{\sup}$
\end{document}