Since the post above was your 20th that is likely the problem when you tried to post there with only 19 post... https://www.backyardchickens.com/t/250783/buy-sell-trade-rules-must-read-agree-to-participate-in-bst
If you put a lot of posts on in a hurry, sometimes the computer takes awhile to catch up with totals.
If you haven't had any luck posting on there now that you have 23 posts, send a PM to one of the moderators explaining the situation and ask them for help. A couple of the names of mods I can think of right off the top of my head are Terrilacy, sumi and TwoCrows.
Welcome to the BYC flock! We are glad you joined us! Any luck getting the buy/sell/trade forum to cooperate?