Accidentally deleted the admin user

You can create a new administrator account by restarting the Apple Setup.

Even though this Setup Assistant only runs once when you first install Mac OS X / macOS, it remains on your computer and is prevented from running by the presences of a hidden file called <.AppleSetupDone> in a system folder on your Mac.

Therefore, to have the Setup Assistant run again so you can create a fresh admin account, you simply need to remove this file

1. Boot into Single User Mode: Start/restart your Mac. As soon as you hear the startup tone, press and hold ⌘ + S
2. until you see a black screen with white lettering. (If you end up back
3. on the login screen after a flash of the black screen with white
4. lettering, enter your password and it will return to the black screen.)
5. Check and repair the drive by typing /sbin/fsck -fy then ↩ enter - as directed by the on-screen text.
6. Mount the drive as read-write by typing /sbin/mount -uw / then ↩ enter.
7. Remove the Apple Setup Done file by typing
8. rm /var/db/.AppleSetupDone then ↩ enter.
9. Reboot by typing reboot then ↩ enter.
10. Complete the setup process, creating a new admin account.

ref: https://apple.stackexchange.com/questions/164331/i-dont-have-administrator-account-on-my-mac

ref: How To Recover Lost Admin Rights On macOS High Sierra and Sierra, Mac OS X El Capitan And Yosemite - miapple.me