useradd is a low level system utility which by default doesn't do much more than create a new bare bones user account, to get it to create a usable user account you need to specify a number of switches to set things like a home directory etc. adduser is a much more user friendly higher level tool that by default will create a "normal" user account. Give that a go and let us know.

Also keep in mind that TurnKey does not have sudo installed by default so your new user will not be able to do much until you have set appropriate permissions using your root account. If you want to use your new account instead of root then you'll want to install sudo and add your user to the sudo group.