Do you have Tableau Server? If so, enter the server that you use on your machine and the username and password.
I have Pro. So I can't log into the Tableau server?
Tableau Server is a different product than Tableau Desktop Professional (although, it's necessary to have Tableau Desktop for Server).
The Tableau Mobile app only works with Tableau Server currently. I'd recommend reaching out to your Sales rep for more information.
Hope this helps!