Hi ,
I'm Dyari. Thanks for reaching out. I will be happy to assist you in this regard.
If your account is not an administrator, you will not be able to add new users or make account level changes. For that, you need to sign in with an admin account on the PC. Is there any other admin account on your computer?
For the other computer that got locked, try restarting it first and sign in with the correct existing account. If you cannot get past the sign-in screen, use the password reset option if it appears, or ask the person who manages that computer’s admin account to unlock it.
Please keep me posted.