Can you tell what does the error message say? Is it a code 400 or 403 error? Also make sure you pass valid .ROBLOSECURITY cookie as the authorization.
By the way I would recommend you to use some lib which is able to do that kind of stuff. I don’t know about ones for php, but noblox.js is a great lib for node js which I use for stuff such as uploading models. It’s fairy simple to use it too.
If you are going to consider using it, I suggest you check out this reply under another thread which shows an example code for changing somebody’s rank in a group with the noblox.js lib.