Windows 10 Dell Optiplex Administrator Account

I have a Dell desktop that when boots up shows to account available to logon with. User "Katie" and "onlineuser" . We don't know the correct password for either of these accounts. I am able to enter the BIOS and assign a password and activate the admin account but it never shows up on the login screen just the two accounts above. I know the admin password but can't ever get it to let me use the admin account. Even when booting to safe mode I can't use the admin account. What's the next step.

 

See if this helps:
https://www.howtogeek.com/222262/how-to-reset-your-forgotten-password-in-windows-10/