When can I enter the playing area?

An assistant referee may enter the playing area to report foul play, but only at the next stoppage in play and when the referee allows (Law 6.23).

After you signal foul play, you stay in touch and continue all your other duties until the next stoppage in play. Then, at the invitation of the referee, you may enter the playing area to report the offence to the referee, and the referee takes appropriate action.