Football Manager 2020

I was having trouble with this achievement. What worked for me was starting an Online Career in my case I used Chelsea. My 3rd friendly was against Portsmouth so I added a new head coach to manage them. During team selection I picked my best players and for Portsmouth I picked youth prospects and set both goalies as the strikers and their striker as goalie. Once I won the game the achievement popped.

I got this to pop by playing a friendly match against my reserve team in which I managing both sides.

When arranging the match just check under staff responsibilities that you are in charge of both 1st team and reserve teams if not delegate it to yourself to make sure you get the achievement.