I have modified some details in the text of the file and now I am getting this error. I already installed the corresponding package and nothing ... :(
The error occurs on the line below:
\item $p \to q \,\vdash\, (p \to q) \, \vee \sim r $
Full Code:
https://drive.google.com/open?id=1z27i6UkOAugZBIUpB465lgM9Hxp4cDe9
l3backendpackage through your TeX distribution's package manager (TeX Live'stlmgror MikTeX Console, make sure to update all installed packages as well, if you are using a mutli-user MikTeX installation update in both Admin and User mode). For future questions please don't link to external download sites for the full code example, instead provide a small example document directly in the question (https://tex.meta.stackexchange.com/q/228/35864). – moewe Aug 03 '19 at 04:44