Thank you for that information.
So you are not using the built in admin user then.
I think this is a corruption issue. I am afraid you will not be able to simply convert it in this case, but you will have to create a new local user and migrate your files over.
To do so,
- Type netplwiz in the search on the taskbar and hit enter. When launched, click on "Add". On the next window, click on the bottom where it says to create an account without Microsoft email. This will allow you to define your own username. Make sure it doesn't contain space or special character. Set it up as a local user.
- When the above user is created, back to the main netplwiz page, double click on this new user, go to the Group Membership tab, make it administrator. Save the change.
- Restart the computer. Sign into the new user you created in Step 1.
- If you have data at the old username folder, go to C:\users\old username folder, find them and copy them into C:\users\new username folder.
- Type sysdm.cpl in the search on the taskbar and hit enter, go to Advanced tab, click on the "Settings" button at the User Profile section. Then delete the old user profile.
- Launch netplwiz again, delete the old user from there.