What do I do when there is no Admin on my Mac?
How do I get an admin back onto my computer? Do I need to go to an Apple Store?
MacBook Air 13″, macOS 10.13
How many user accounts are there on the Mac? The first one is always an Admin account and I'm sure you can't delete an Admin user account if it is the only one on the Mac.
You can check this in System Preferences > Users and Accounts. The status of each user account is given under the username.
This support document describes the different types of Mac user accounts.