What you’re sending in Postman is different from what you send using RF. What you can try:
${actions_list}= Create List sa.pia.is.w
${body}= Create Dictionary action=${actions_list}
Also note that data validation on the endpoint is lacking if you receive a 500 response on invalid data (in your case, a string instead of a list of strings). A typical response would be 422 in such a case.