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.
Related LaTeX fixes
Browse all LaTeX problems & fixes, or start with the LaTeX guide.
Try the fix in LetX
Open the editor, paste your code, and compile in seconds to see the error clear — free, in your browser.
Open LetX Free