Delete admin account.

In the Apple menu in the menu bar, you’ll find a logout option.

It could be inferred that this is a new-to-you Mac and you now wish to set it up for your use. If so, then this current approach of swapping user accounts won’t get you where you want. The installed software and apps will still be associated with the previous owner, and you won't be able to update those without the previous owner’s Apple ID and password. Among other issues.

If this is a new-to-you Mac, then here is the sequence that Apple recommends following:

What to do before you sell, give away, or trade in your Mac - Apple Support

le_giulia Said:

"Delete admin account.: [...]I can't delete my original admin account. I saw the posts on the community and I saw some videos but it won't work. I noticed that in my case I can't click on the account that I want to delete, and there is this little v sign in orange. How can I resolve the issue?[...]"

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Create Third New Administrator User:

Create a new, third, Administrator user, and see if you can delete it using the new administrator. Once deleted, then rid of this third administrator user. You should be good from there.

MrHoffman Said:

"A third administrator login should not be necessary here, so long as the deletion is aimed at the other of the two admin users."

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Why a Third Administrator?:

A setting in one of the other two Administrator Users may have become manipulated, technically, in one manor or another.

So, a newly-created Administrator User wouldn't have all those manipulated settings (should that be the case), overruling this account deletion.

Deleting a logged-in user leaves the state of things... messy.

Self-deletion gets messy, too.

Deleting a another user—one not presently logged in—is typical and expected and works.

If that deletion fails, then more diffing is warranted.

le_giulia wrote:

I tried to disconnect everything: photos, iCloud, music... But still it won't let me delete my first admin account...

To confirm... This is after having wiped and re-installed a fresh copy of macOS, and that new install is under your Apple ID, and when you are logged into the second Admin user, and when the first Admin user is not still logged in when Fast User Switching is enabled?

Your Mac is required to have at least one Admin account at all times.

Log in under the other Administrator, log out the first, and then try the delete again. Deleting the logged-in user would be bad.

A third administrator login should not be necessary here, so long as the deletion is aimed at the other of the two admin users.

You are welcome.