Profile: steveL-12

Problem:
How can we add Repeatable Field Groups to a Post using the REST API and Toolset?

Solution:
You can set repeating field groups the same way as you'd set a post belonging to another post.
https://toolset.com/documentation/programmer-reference/toolset-integration-with-the-rest-api/

Since RFG (Repeatable Field Groups) are in fact Posts, gathering a set of fields, you'd need to threat the RFG itself as a Post (and that RFG will belong to the Post where it's added to).
The Fields within the RFG are to see as custom fields like when added to a post type, just here the post type is the RFG.

However, to "get" RFG's, to have this incorporated in Toolset you'd have to submit a Feature Request for it in https://toolset.com/home/contact-us/suggest-a-new-feature-for-toolset/

For now, you'd be able to add your own custom endpoint, if you are versed with the WordPress REST API which provides API endpoints for WordPress data types, you could do that, but we can not assist it here in the Support.

An example of adding such an endpoint, using a post-relationship as an example with "customendpoint" namespace can be seen here https://toolset.com/forums/topic/writing-rfg-to-cpt-via-rest/#post-1310253