This tool currently works for Fifa 20 webapp, will try to support when Fifa 21 is released.
Installation And Demo - https://www.youtube.com/watch?v=M3Jn4FI1nUw
Fifa-AutoBuyer TamperMonkey Scripts
Add Tamper Monkey Extenstion To your Browser. Then Navigate to Create Script -> Then to Utilities Under Utilities -> File (Import the two Scripts and Install it). In Ultimate Team Web App, New Menu will be added as AutoBuyer.
Enjoy...