This is most likely caused by the much-dreaded "font-not-embedded" issue with some PDF/EPS graphics in your project, because the application that created them did not embed the fonts in the fonts: See this link for some background details (especially item no.2).
Regarding the fonts: Here the culprit most likely are graphics, either in eps or pdf format. If you have a graphic with text which does not also contain the font, the font will be missing in the document as well. You have basically two choices here: a) In whatever tool you are using to create the graphics, look for an option to explicitly contain the fonts into the exported graphics. b) Export the text not as text but as curves. If there is no text, no fonts can be missing (ugly solution).
The best solution is to ensure that your PDF images were exported from their original applications with the embed all fonts option. For example, if you're generating images with R, you can use the
embedFonts method (also here and here).
Alternatively, you can get Overleaf to post-process the PDF for you, though this may cause your project to take some longer time to compile. First, add a file called
latexmkrc (without extensions) to your project, and then paste the following line in that file:
$pdflatex = 'pdflatex %O %S; ps2pdf14 -dPDFSETTINGS=/prepress %B.pdf %B-embed.pdf; mv %B-embed.pdf %B.pdf';
You may have to make some kind of small change in your
.tex file to trigger a re-compilation and the post-processing to happen.