Wait until the ball touches the grass to increase first-time shot accuracy
Volleys are tricky in PES 2021, even if your player has the specific "First-time Shot" skill. You will find more success if you let the ball touch the grass before shooting.
In this clip, my player is unmarked and facing the goal, which are two great signs, but I'm still pretty sure the ball wouldn't go in if I didn't wait for it to touch the grass first.
To make sure the player does not attempt to get a hold of the ball, make sure you use Super Cancel (R1+R2) to control your player and wait for the right time to shoot.
When I see I have enough time and space, I always prefer this option to improve shot accuracy.