Go to your listing ("ad") page, go to the bottom where you have the options to save, preview, post, etc., and click the button for "error check." A message will appear at top of page telling you either there are no errors (green message) or that there is an error (red)--in which case the errors are explained in plain English with instruction on how to fix them. You can re-check for errors after "fixing" and then if it's okay, save.