A user object cannot be used if you wish to interact with roles because it lacks roles. The roles property is contained in Guild Member, which you must use.

All you need to do is modify your code to: let usera = message.mentions.members.first();

instead of message.mentions.users.first().

Edit: Take note that this implies that in order to make the user portion of your code a user object, you will need to enter usera.user.