@Marianne25 , sounds like you have a complete reservation. can you check in your Airbnb confirmation and look there to see exactly how much you were charged? If it is the amount without the credit, I suspect (don't know for sure) that as you say, they don't let you use your credits for every stay. In that case, I imagine it will show up again when you do your next reservation.
But for peace of mind, yes, calling Airbnb might be helpful.
I have heard that using twitter @airbnbhelp works real well.
Also for @Stephen189 and @Rav1 , if you look here in the forum under Community help, the first post is all about how to contact Airbnb:
https://community.withairbnb.com/t5/forums/v3_1/forumtopicpage/board-id/Community-Help-EN/page/1/thr...