That's because you're still new here. The policy is once you have 10 forum posts, and have been here for 30 days, then you can start posting in the marketplace. It's that way for 2 reasons. 1, to help stop spam, and 2, to get people to actually take part in the site, and not come here for only the marketplace, since this is suppose to be a community site, not etsy.
Also, posting a commission wanted ad isn't the best way to get a commission. The BEST way to make sure people see that you want a commission is to directly message the commissioners. Nothing is stopping you from that.