You have created a new Comment type, but when you try to add a comment...

In Drupal, a comment field can only use comment types that are configured for the same target entity type as the entity you are attaching comments to. When you add a comment field to a Content type (node), Drupal expects a comment type whose Target entity type is Content . If the new comment type was created with another target entity type, such as custom block or user, it will not appear as a selectable option when adding that comment field to a content type. This is consistent with Drupal core’s comment architecture and field behavior.

The other choices do not match Drupal’s documented behavior. “Allow comments” is not a separate prerequisite checkbox that controls whether a comment type appears; instead, comments are enabled by adding a comment field to the content type. Drupal documentation for administering comment settings confirms that comment capability is added from Manage fields by adding or reusing a comment field. There is also no documented requirement that custom comment types work only on default content types, and cache clearing is not described as the fix for this selection issue.