If you receive the "This experience is locked for player 2. Their parent or guardian needs to unlock it from their account while not in split screen" error while trying to play Battle Royale mode despite not being in a party and not playing on split screen, try clearing your console's cache to troubleshoot the issue: How do I clear or reset my device's cache?
If clearing your device's cache doesn't help, you can try changing your DNS settings: How do I change my DNS servers on my platform to connect to Fortnite?
Note: This error is not the same as "This experience is locked. Your parent or guardian needs to unlock it". For this message, please refer to Fortnite - Can't access event or game because it says it is locked