Try a pi2 app

How can I try(before buy) a pi2 app if the bui;d button is disabled?

Press Run?

I’m not sure if Remote Debugging works if you don’t have a license?
If it does you can use that.

Question is what kind of PI App?

The PI Console (“based”?) Apps - you can get the License for Free.
Only if you want to build “Desktop”-Apps for Raspi Linux you would need to pay for a License.