What an odd thing for it to say, especially since if you can’t select a character to post, how are you supposed to post? O.o
I assume you are using your phone to be able to post?
This has been an ongoing issue for a few months now, which we’re aware of, but there isn’t anything that we can do through this forum about it. Other than bug the folks working on it.
The only advice I have is, you can try logging in and out of the forums to see if that refreshes the account status but until an overall resolution can be implemented it seems to be an issue that comes and goes.
This has been a rather persistent bug that, as I understand it, has to do with some other systems. Once some work with those systems completes, which I understand is a bit of a process, this should stop happening.