Listings package language Assembler could not be loaded

If you don't select a language dialect in square brackets, listings falls back to the default dialect and if there is none to the empty dialect.

The language Assembler is defined without an empty dialect and has no default dialect, so you need to explicitly specify a dialect when you select the language. The available dialects of Assembler are x86masm and Motorola68k.

Remember that LaTeX usually only accepts nested square brackets when you hide the inner brackets in curly braces.

documentclass{article} 
title{Bind TCP shell} 
usepackage{listings}

begin{document}
maketitle
section{How to call a syscall on linux in ASMx86}
paragraph{Teory}
paragraph{Example}
begin{lstlisting}[language={[x86masm]Assembler}]
mov eax,edx
int=X
testo
end{lstlisting}
testo
end{document}