I am working on a LaTeX document where I need to use the citeonline command for in-text citations. However, I’ve encountered an issue where this command does not seem to exist in some of my LaTeX files, causing compilation errors and messing up my PDF output. This problem does not occur in all my documents, only certain ones.

thats my packages

usepackage[brazil]{babel}
usepackage[utf8]{inputenc}
usepackage{natbib}