On the details page for a webapp during the submission flow, the Support Email field is marked as required (and the requirement is enforced), although the description of the field makes it sound like the field is optional: If you wish to display an email address for support inquiries, enter it here. Either it should be an optional field, or its description should be sterner. ;-)
https://github.com/mozilla/zamboni/commit/cbb9878 Help text is now sterner. This will go live on marketplace.mozilla.org on Thursday, 3/29. Thanks!
verified fixed at https://marketplace-dev.allizom.org/en-US/developers/submit/app/details/seavans-undersea-adventure--4