ios 15 can't open pictureinpicture using startPictureInPicture

I can use ios16 to display floating windows, but ios15 cannot. Why

@try { NSError *error = nil;

[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayback mode:AVAudioSessionModeMoviePlayback

 options:AVAudioSessionCategoryOptionInterruptSpokenAudioAndMixWithOthers
 error:&error];


[[AVAudioSession sharedInstance] setCategory:AVAudioSessionOrientationBack error:&error];

[[AVAudioSession sharedInstance] setActive:YES error:&error];

} @catch ( NSException *exception ) { NSLog( @"AVAudioSession error" ); }

self.pipVC = [[AVPictureInPictureController alloc] initWithPlayerLayer:self.playerLayer];

self.pipVC.delegate = self;

[self.pipVC setValue:@1 forKey:@"controlsStyle"];

if ( ![self.pipVC isPictureInPictureActive] ) { [self.pipVC startPictureInPicture]; }