If the software's configuration has become corrupted, resetting can often bypass the error. Go to the Windows Start menu.
If the above steps fail, you likely need a clean install. This is more thorough than a standard uninstall. Remove AutoCAD 2021 via the Control Panel.
Even if the file is present, Windows may have "locked" it because it came from another computer (a flag applied during installation).
Have you checked your to see if it quarantined any Autodesk files recently? Share public link
