So, there has been an issue that keeps popping up on the WoW Forums where folks are being told that they need a level 10 character to post even though they have multiple characters that qualify.
This issue has been happening for the last 2 months or so but comes and goes. It has been under investigation and they are working on an overall resolution to stop it from happening going forward, but we don’t have an ETA on when that resolution can be implemented.
I believe the notice that you cannot post until (X - date from the past) is part of that bug. You haven’t been recently suspended, it is likely picking up old data.
I’m afraid there is no workaround that seems to help here. If you are unable to post because you get the level 10 requirement one or the "you’ve been suspended until sometime in the past, you can try logging into and out of the forums a few times, or reset your password, change browsers, try private mode or a different platform (PC/Mobile), but ultimately, until a more permanent resolution is found, you might just have to wait until the ability to post returns on its own.