Problem: I am trying to place a slider gallery block inside a Content Template and use a custom field as the dynamic source, but I cannot select the custom field because the field group is not an option in the dynamic sources panel. The Content Template where I want to place the gallery is not assigned to the post type containing the custom field with images, it is placed in a View, which is also nested inside another Content Template.
Solution: Temporarily assign the Content Template containing the gallery block to a post containing the custom field with images. Then choose the correct custom field in dynamic sources, and verify the field is displayed correctly on the front-end of the site. Once you have finished designing the gallery, unassign the Content Template from the post.