There's currently some migration happening with the invitation system. All you can really do is wait. If it's been more than 3 weeks, please contact support. Also ensure you have monetisation enabled on your YouTube Features page (yes, this may require AdSense), no strikes and not many claims on your channel.
they are currently having a lot of requests to the network from what I've heard, I'm in the same boat, just give them some time. If it goes over a week or two then contact support to see how far back in the queue you are.