Check if you booking code is also valid for TAP. You can find TP code on your itinerary, if not, you can access with last name and ticket number. If your ticket has been issued less than 1 day ago, wait 2 more, you will be able to access it then.
If non of the above works, call the agency that issued your ticket!
If your TP leg is code share (LHxxxx) number then you can only reserve seats through TAP!