Did you encounter this with the new b4a v5 beta? If yes; mark your thread with "b4a beta" please.
Check if you start b4a.exe now. In earlier versions it was basic4android.exe
I ran into the same error...
Solution:
Do one time a "Open with ..." and select the b4a.exe and mark the checkbox if you want to use the new defined program for opening b4a files.