If you're still facing difficulties or receiving an error message, then the below troubleshooting guide should be your next step (Source info courtesy of iDB □). Just a heads up: It can take up to 24 hours for iMessage & Facetime to activate. These steps should be enough to get the iMessage setup on any iPhone.
Send & Receive (Set your mobile number here along with your email address)
Settings -> Messages -> iMessage (Toggle switch to ON position)