Let me try and help you. Most exchanges are currently not yet accepting deposits/withdrawals of Steem as they are sorting out everything on their side. Nothing much any of us can do but wait. You can use https://ionomy.com/ but the price is super volatile there due to low volume.

The Missing Posting Authority is an error that gets send and does not mean you are not allowed to post. It just means that there currently was a problem matching your posting key. I see you managed to made this comment, so I assume this is resolved? If not please let me know what you are using to log in at the moment.