This is unfortunately a common error message however, there's really not much assistance the community forum can provide. There's nothing we can fix. The message can be because of various reasons, i.e., it could be a system glitch, it can be because of an unverified account with either the seller or buyer, etc. We really don't know as no one here has access to user account info. Sometimes the issue simply goes away, other times, not.
You can try deleting your web browser's cookies, temp files and history, then retrying, else, you can try contacting customer support direct for assistance.
