I am working on PvP football game in Unreal Engine. Currently working on AI for the game. Basically goalkeepers and other pawns who are not controlled by players. I am a little familiar with Unreal’s AI system, but I don’t know how to approach this.
Football is hard.
Start with the design. You’ll have to figure out how the AI should behave, first on team level and then on individual level.
To be honest most football coaches struggle with that 
The ones that good at it are payed a lot.
Start with the basic rules you are most sure about and go from there.
On the technical side try to implement move, pass and shoot. Then figure out what are the rules for each of those.