Are you sure you can have a “json” field type??
The official documentation doesn’t state that “json” is acceptable.
Use a “text” type instead.
For more information, see Parameters | Make Apps
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!