How to remove one administrator account in Catalina

If the account is greyed out it normally means it's still logged in. Log out of the current account, then log out of the account you are trying to delete. Then log into the account you want to keep and try deleting the second account again.

Delete a user or group on Mac - Apple Support

Bjorne Said:

”In the transition and moving everything from my old iMac to a new Mac mini I have managed to create two Administrator accounts with a little different contents on the Mini's HD.[...]”

———-

Try this:

Transfer it All — Manually:

First, Reformat the Hard Drive, then Transfer it all over:

1. Reformatting the Hard Drive:

Start by exporting all documents to an external drive. Then, rid of a user. If that is of no success, then reinstall the Mac, erasing the entire hard drive before the installation. But, for the sake of losing a successful installation, back up your Mac using Time Machine Backup, prior to the reformat.

2. Transfer it All - Use Migration Assistant:

Once reinstalled, use Migration Assistant to move over the documents and such.

transfer it all over using migration assistant. Go Here: How to move your content to a new Mac - Apple Support

John's reply reminded me, you do have to unlock System Preferences. I assumed you had.

A quick word of advice: it is always a good idea to keep a second administrator account. Should there be a problem with your main account, that second account may be a godsend.