"An exception occured" error is given when something goes wrong at the end of Mercari.com or Poshmark.com. This error is temporary.
How to resolve this error?
This error will be solved by itself when you try again after some time. When it does not get solved for a long time, try to add a new listing manually and get in touch with platform officials (from Mercari or Poshmark) if you still get the same error.