KeyNotFoundException: The given key was not present in the dictionary

Hello, so basically am new to unity and specially for uNet, I wanted to create a multiplayer game, I followed brackeys tutorials, It was going so good until I reached episode 7, In there When he did the damage part I did what he did, but then I got the following error : KeyNotFoundException: The given key was not present in the dictionary, I searched On internet and even went to the video comment section but no body talked about that, Hopefully I find a way to get rid of this, and here is the code problem:

    public static Player GetPlayer (string _playerID)
     {
         return players[_playerID];           // When I click the error it takes me to this line.
     }

The error means that the player with id of _playerID wasn’t present in the dictionary. This is probably because you forgot to add that player to the dictionary. Check that and if that doesn’t work could you share your other code as this is not the area where the real problem is.
@duck dyari_dlshad