The document compiles, but there is a crazy amount of warnings and errors happening in the background. Overleaf somehow fixes these errors on the fly, but the "code" wouldn't compile outside of Overleaf because of these errors. We need to fix them.
A good read related to this issue: https://tex.meta.stackexchange.com/questions/7898/examples-of-overleaf-users-not-noticing-error-messages