Delete a standard user from Mac OS
The – button is greyed out for a user when the user is still logged in with Fast User Switching. Log out the other user you're trying to delete, then return to System Preferences with this user account and try again. You can restart to be sure that user is inactive as well. 

You can delete a user from command line and you need to be careful you have the exact short name correct or you will erase the wrong data. In this case, the short name is mac. Run the following to delete the user and separately delete the user's home folder. If you want to keep the home folder, don't run the rm command.

sudo dscl . delete /Users/mac
sudo rm -rf /Users/mac


If the dscl command fails, reset the password on the account and retry the above commands.

sudo passwd mac