We have identified a solution to the issue preventing Steam users from purchasing Heavensward if they played the FINAL FANTASY XIV: A Realm Reborn free trial on Steam and upgraded to the full game via the Mog Station. However, we will need users who are affected by this to contact the North American Square Enix Support Center so that we can assist them. If you are affected by this issue and are unable to purchase Heavensward on Steam, please visit our Support Center and follow the instructions listed to receive assistance.

Assistance for pre-ordering Heavensward on Steam
http://sqex.to/ys9

We apologize for the inconvenience and thank you for your understanding.