I just received the great engenier, but cannot assigned to station. It said station locked. Any idea..

Do you have enough Cost Capacity for stationing more heroes? tap on the Management Station building in your city and then on 'Station' to check the 'Stationing management' screen.

This shows you what heroes are stationed an how much capacity you have (at the top) Each hero will need a certain amount of points to station. Orange Heros will need a lot of points.

Upgrade the Management station building to increase the capacity.