Thank you @Edison peng. I solved the same issue by passing in Content-Length: 0.
Wish all of you have a nice day :D
I run into the same issue. The status code is 411 Length Required.
After adding header "-H 'Content-Length: 0'" in my API request, line accepts it without error.
Can you get created rich menu id via Get rich menu list API? https://developers.line.biz/en/reference/messaging-api/#get-rich-menu-list
If you get it, pleace share more detail set default richmenu API response(status code and detail).
Could you share the error message?