From: Lower Saxony Germany
Hi Rainer it might be than you don't have enough AP, supplies or raw to build an airfield or airbase. When you click on the build button a page should open. Left side is listed what can be built on the right side what "materials" are needed. If you have enough you will see ok, if you are lacking then short will appear.
I hope I could have helped you. If you require more help send me a PM in German if you want.
"It is not enough to expect a man to pay for the best, you must also give him what he pays for." Alfred Dunhill