If you have just logged into the new system for the first time, you must wait for CRON job to run to set eligibility. Another possibility would be you are trying to post in a category that has been deprecated. List of deprecated categories in buy, sell, trade is found here https://forums.digitalpoint.com/forums/deprecated.96/
@djixas I think DP takes scheduled update automatically or manually in programming sense. After update, you will get PM regarding "welcome as a Established member" then you will be able to post in marketplaces. @ Smyrl Am I right?