Jesus told His disciples this after He rose from the dead and the devil had been completely defeated. God doesn’t have to let us have free will, He does it because He loves us and wants us to choose Him and His ways. He has the authority to take these choices away, but He doesn’t.
The the thing the devil can still do, because God gives us freedom, is to influence those choices, to distract us from prayer and God’s Word, and make us too busy to notice. This spiritual battle is always going on in and around us - make sure you’re paying attention. Jesus has all authority but we have to choose to live under it to reap the benefit. Is there something in your life that you haven’t put under Christ’s authority?