Hi,
I have a question, because when I try to add component, in this case “Network Manager” i read a message like " The high level API are deprecated and will be remove in the future". I don’t know what it supposes to do and I can’t add “player prefab”.
What package are you using? If you’re using HPLAYI multiplayer package from the Unity Package manager, it was deprecated long ago, meaning all scripts are obsolete and don’t work any longer. I suggest either looking at Mirror or Photon Pun, Photon Classic or other packages, or using Unity Multiplayer services instead.