I was having the same problem, then I realized that the account I was trying to delete was logged in. (If you click on the user's account name in the top right corner you can see an orange checkmark appears beside all the account that are currently logged in). So I went back to the account I was trying to delete to make sure I logged out (this by clicking on the apple at the top left corner and selecting "log out USER" or press Shift + Cmd + Q. Then you can go back to the admin account you want to keep, and now the subtraction button and the account you want to delete should be clickable 🙂 I hope it works, thank God it did for me 😀