On the 'Troubleshooting issues with the email channel' documentation page (under 'Receiving requests by email', 'Administering service desk projects') in the last row of the table, there's advice on what to do when you get the message that there are no suitable request types. It says you need to add a new request and make both the Summary and Description fields Required. However, it's not necessary to make these fields required, they only need to be visible.