On the ship page, to the left, you can see the icon of the pilot.
1) Is the pilot activated (does he have a level? Or does he appear stripey and without a number indicating his level)?
2) Do you have enough blueprints to activate the ship?
3) Do you have enough ship materials (credits) to activate the ship? Can't remember how many you need, it wasn't a lot, but check the top right corner to see if the number there is greater than the number it says you need.
If one of these is missing, you can't activate the ship. If the game won't let you leave the screen to complete one of these things you'll have to contact CG support.
The most likely situation is that you don't have Tie Fighter Pilot activated. You can click on the pilot's icon (left) to go to the activation screen for the pilot.