Thanks for the answer. I already added a reference, but it only worked after restarting Android Studio. So this problem is solved. But there's still the problem that I can't import anything. Everytime I finished my "import line" it just disappears and I get the following error: Access is allowed from event dispatch thread only.
Details: Current thread: Thread[JobScheduler FJ pool 5/8,6,main] 1703161454
Our dispatch thread:Thread[AWT-EventQueue-0 0.5.0#AI-134.1058404, eap:true,6,main] 357586160
SystemEventQueueThread: Thread[AWT-EventQueue-0 0.5.0#AI-134.1058404, eap:true,6,main] 357586160
I really get this error when I want to import anything and not only when I'm importing "com.google.android.gms.ads.*"
Is there any solution for this problem?
Thank you