Botting HTTP requests

Hello, I've been recently trying to bot an iOS game called Monster Legends but I'm not quite experienced in HTTP botting so I'm kinda lost in here.
Here's what I've done:

I connected my phone through a proxy in order to monitor it's traffic with Burp Suite with my pc.
Now I can perfectly monitor every single GET or POST request this game handles, but when I try to repeat one of the requests that my phone POSTS to the game's host I come up to this error:



Apparently I can keep sending the request to the server, but it just wont recongize it. Any ideas on how to modify the request so that I can make a POST bot?

probably u can sniff and edit packets but u can't create new