If you changed your bibliography package from a BibTeX-based solution to
biblatex or vice versa it is not at all unlikely that there would be some errors due to left over auxiliary files. BibTeX and Biber use these files to communicate with LaTeX (see Question mark or bold citation key instead of citation number for a great explanation), but the format of that file and its precise usage differs between
biblatex and standard BibTeX. If the
.bbl was produced for one method it will not be usable for the other. Usually
latexmk (the tool Overleaf uses to compile your TeX document automatically) is quite good at sorting out issues like this, but sometimes it gets stuck with old file versions. In that case it helps to clear the auxiliary files (Overleaf calls this 'clearing the cache'). It is generally safe to delete the temporary files and recompile from scratch.
You can clear the auxiliary files via the following steps.
Click on the 'Logs and output files' button next to the Recompile button on the top of the rightmost pane, it will probably show the number of errors in a red box (highlighted in light pink in the screenshot).
You will be greeted with a digest of all error messages and warnings that LaTeX and other auxiliary programs like BibTeX or Biber reported. Scroll down to the bottom of the page. On the bottom right you'll see a little dustbin icon (highlighted in blue). Its hover-text says 'Clear cached files'. That button allows you to get rid of all temporary auxiliary files (
A popup messages asks you to confirm.
If you press "Clear cache" in that window, Overleaf deletes all your temporary files. You can then recompile and the error should be gone.