LaTeX Error: File `xxx.sty' not found
The "File `x.sty` not found" error means LaTeX needs a package that is not installed. Fix it by installing the package — run tlmgr install name on TeX Live, or use the MiKTeX Console — or by correcting a misspelled package name. Online editors like LetX and Overleaf ship every CTAN package, so nothing needs installing.
Why it happens
Each \usepackage{x} loads a file x.sty. If that file is not in your TeX distribution — not installed, misspelled, or a custom journal class you did not upload — the compiler stops with "File `x.sty` not found". The exact missing filename is printed in the error.
Example
% Error: File `siunitx.sty' not found.
\usepackage{siunitx}
% Fix on TeX Live (terminal):
% tlmgr install siunitx
% Fix on MiKTeX: MiKTeX Console > Packages > install siunitx
% Custom template .sty from a journal?
% Upload it into the same folder as main.tex.Frequently asked questions
How do I find which package a .sty belongs to?
On TeX Live run: tlmgr search --global --file name.sty. It prints the package that provides that file, which you then install with tlmgr install <package>.
The .sty came with a conference template.
Custom classes and styles (a journal .sty, IEEEtran.cls) are not separate CTAN installs — put the file in the same folder as your main .tex, or upload it to your project.
How do I avoid installing packages at all?
Use an online editor with a full TeX Live distribution. LetX and Overleaf preload every CTAN package, so \usepackage just works with no install step.
Try the fix in LetX
Open the editor, paste your code, and compile in 1 second to see the error clear — free, in your browser.
Open LetX Free