The operation "Add" is missing on the Translate page of a menu item

I see in the code that the route for Adding translation is there and is getting the custom permission set:

case 'entity.menu_link_content.canonical':
        case 'entity.menu_link_content.delete_form':
        case 'entity.menu_link_content.content_translation_overview':
        case 'entity.menu_link_content.content_translation_add':
        case 'entity.menu_link_content.content_translation_edit':
        case 'entity.menu_link_content.content_translation_delete':
          $route->setRequirements(['_custom_access' => '\Drupal\menu_admin_per_menu\Access\MenuAdminPerMenuAccess::menuItemAccess']);
          break;

So I'm not sure why the "Add" button is missing next to menu items.

I just figured it out. It turns out you need to enabled the permission "Manage translations for any entity that the user can edit".