The reason why you should not use Admin as your username on any WordPress installation is very simple;
It is the default WordPress user name.
Hackers are of course well aware of this, and the admin username is likely the first one they will try out to be able to gain access to your site.
It’s much better to use your first name or maybe a combination of your first and last name. This can of course be guessed quite easily if someone is aware of you being the owner or user of the site, though bots are unlikely to have that information available to them.
If you want a very safe username, then we would recommend that you generate a random string using a password generator like Strong Password Generator.