In Blocks 1.5 we introduced display conditions to Content Templates, so that you could—for example—create a template for posts that should only be used for a particular category.
An unintended side effect has been to break Dynamic Sources in such templates, so that you cannot—for example—select fields from parent posts as the source, or select taxonomies or custom fields from field groups assigned to the post type as a source.
For now, it is necessary to remove any display conditions from the template while making such changes (and to do that you must unassign the post type and then re-assign it without adding any conditions). After making your changes you can add back the conditions.
The dynamic sources will work on the front end, even though it is not possible to insert blocks with such dynamic sources in the back end if the template is conditionally displayed.