To change a user from standard to admin, you need to be an admin user. For example, the command “dscl” can do that in Terminal, but has to be run using “sudo” to have the required privileges. Similarly, in Users & Groups, or the Directory Utility in CoreServices, you would need to authenticate as an admin user in order to make this change.
As your standard user doesn’t have the privileges to do that, the best approach is to perform a clean install of macOS and migrate from the last backup before you changed that account to a standard user.