You might be having a problem posting an ad because your description has a phone number or email address, which is not allowed. (That's, of course, if the ad you're trying to post sticks to our Posting Rules.)
Rather, add your contact number in the space provided when you create an ad. That way buyers no longer have to copy and paste your number to reach you. They simply have to tap on a button.
Still struggling? This article describes common issues around posting an ad and the resolution might be there.