you cannot use a default admin account. if this is the only account on the machine, create a new user, and follow the steps once logged into that user.

you will also need a domain admin user from the domain to authorize the join

go to system properties > bottom right > change > input a name for the computer, cannot be the same as others on the domain > beneath the name, select domain and type the name of the domain you wish to join

a login will put up for authorization. use the login of the domain admin , not the local admin, or user you are currently logged into

you will need to reboot.