In this case, it's likely that the Google bots may have crawled through your site before you fixed the errors and haven't yet recrawled to detect the changes. To fix this issue, you'll need to invest in premium SEO tools such as Ahrefs or Screaming Frog that can audit your website both before and after you make changes. Once you have them in place, take screenshots of the findings both before and after fixing the issues and send those to your client so they can see the improvements that have been made.

To give you an example, I recently encountered a similar issue while working with a medical billing company named HMS USA LLC. After running some SEO audits and making various fixes, the GSC errors had been cleared. However, it took a few attempts to get it right as the changes weren't detected on the first recrawl.

Hopefully, this information is useful and helps you understand why your GSC issues may still be showing up after being fixed. Good luck!