Welcome to the Make community!
You’ll need to use the “Transform to JSON” module to convert the variables into JSON, to be mapped into the Body field of the HTTP module.
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!