# ! LaTeX Error: File l3backend-pdfmode.def' not found (in line)

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:

• It should be enough to install the l3backend package through your TeX distribution's package manager (TeX Live's tlmgr` or 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 (tex.meta.stackexchange.com/q/228/35864). – moewe Aug 3 at 4:44