Unable to return user favorites via Tableau REST API
The approach i would take is:
Query a user on the site. (the user that has the favorites)
Check if the user is actually: the same user you are authenticated as; and the same user you are gonna query for favorites
If they are the same, try adding a favorite with the REST API (DataSource, View or Workbook)
Get the favorites for the user, the datasource/view/workbook you added as a favorite should be in there.
If you want to Update the user, Add user to site or Add user to Group, I've added links to the documentation
You can do these things with Postman/tool of your choice.
What you can also try is ensuring the user that is querying another user (or the same) is a server admin (just to be safe), and making sure that you are a member of the same site of another (or the same) user.
Hope this helps!
EDIT: Maybe you can try adding a new user with group regular to a site, ensuring that you are a member of the site too. Afterwards adding a favorite and getting the favorites for the user of group regular. If that doesnt work u can verify whether its impossible to get favorites for users of group regular as well, besides admins.