Confirm identity on Marketplace message was never sent even after trying everything I could.

On Marketplace, I tried send a message to someone to see if they still had the shoe. However when I did a prompt came up that said to confirm identity on Marketplace. When I put my number in it never. For some reason I could message people if they messaged me first though. Like I put something up for sale and someone sent me a message and I could send them messages but now if I try and send message it won’t work. Ive tried everything but the help center doesn’t do anything