Hey there! Sorry for the troubles.
Di you have a fusion drive? (You can see in Apple > About This Mac > System Report > Storage), it will show what type of drive it is.
This may not be the case, but I’ve seen this error on fusion drives, that requires they be rebuilt, due to possibly being reversed or corrupted some other way:
How to fix a split fusion drive:
https://support.apple.com/en-us/HT207584
In your case just running the Catalina command from recovery (diskutil resetFusion), then reinstalling, (As opposed to erasing from Disk Utility then reinstalling), should fix it.
If it’s not a fusion drive, or you’re getting errors in other apps and/or other users, there maybe something else going on, especially if you’ve already erased/reinstalled, perhaps if you migrated data from another Mac, the permissions got messed up. I just mentioned the fusion article from what I’ve seen and that particular error message, which is pretty specific and seems isolated to Mail, it seems like an extreme measure for such a small issue but I’ve seen it work, despite every other step being exhausted, and if you’ve already resorted to erasing and reinstalling, this might be worth a check, and hopefully saves you more time.
Hope that helps! Let me know how it goes!
