Hi! If this is your device and does not belong to an organization, you could try to use Windows Recovery Environment (WinRE) to re-enable the built-in Administrator account with an official command. From your signed-in standard account, go to Settings > System > Recovery > Advanced startup > Restart now. In WinRE choose Troubleshoot > Advanced options > Command Prompt.
Then run: net user administrator /active:yes, close Command Prompt and continue to Windows, sign out, and you should now see Administrator on the sign-in screen.