Why won't new AirPods connect to my existing pair

For like 4 months I’ve been using my AirPod in my left ear by itself because I lost my right and everyone’s perfect, I just bought the right AirPod same model A2699 and when I put it in the case it makes that sound that won’t connect what did I do wrong? They are the AirPods Pro

[Re-Titled by Moderator]