I build custom ISO Ubuntu 20 according to this article.
After the custom ISO is ready, I install the OS from it, and at first, the error appears:
Check finished errors found in 1 files.
Could you tell me how to find this error and fix it?
I already checked md5sum, I read about it.
Based on what I can discover from looking at this bug report and discussion (Check error with 20.04-desktop ISO) it appears this is just a built-in limitation of the file checking where the efi.img file is modified when it is loaded and that really means it can't be verified. I would advise go ahead and ignore this error! That is what I have done with my Xubuntu installation and I have had no problems at all (since then I have also noticed this error when installing Ubuntu Mate and I also ignored it in that case as well!)