We’ve fixed this bug with MySQL 5.7 (Unknown database type json requested) in Mautic 3.2, to be released on November 30. The fix will only apply to new installations and to installations that don’t have db_server_version in their app/config/local.php yet (this is the case if you haven’t changed the default configuration in Mautic’s UI before). Otherwise you have to change the version in app/config/local.php yourself, as mentioned in Mautic 3.1.2 installed - Plugins are not available.
Apologies for the inconvenience and thanks for reporting this!