linhramen Said:
"that didnt work. I can delete the account I've just made but not the first one (Mac)"
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Try the Following, in the Order Provided:
A. Restart the Mac:
Perhaps something is running in the background, an it needs to quit.
B. Log Out of It:
Log in to the user and then log out. You must not be logged in to the user, in order to delete it.
C. Try Safe Mode:
In Safe Mode, login items do not run, and corrupt system files(i.e. caches) are intentionally reset as a way to fix issues
- Boot: in to Safe Mode: [Hold Down: shift key upon boot].
- Log in: as same user
- Delete: this user
- Restart the Mac
D. Reinstall the macOS:
Nothing would be deleted. It's just that items that have become corrupted or deleted, would be replaced or reinstalled back to where they should be. Go Here: How to reinstall macOS from macOS Recovery - Apple Support. Important: Be certain to create a backup of your Mac prior to all of this. That way, you can have something to restore from the Mac from, should anything go wrong with the reinstall. Once reinstalled, try this once more.