question

Is that written in Unity? If so, the error could be that the success callback function OnDisplayNameUpdate in your code is incorrectly defined.

Here is an example of an API call, with success callback and error callback:

  using PlayFab.ClientModels;
    public void Function()
    {
        var request = new UpdateUserTitleDisplayNameRequest
        {
            DisplayName = Input_nickname.text,
        };
        PlayFabClientAPI.UpdateUserTitleDisplayName(request, OnDisplayNameUpdate, OnPlayFabError);
    }
    private void OnDisplayNameUpdate(UpdateUserTitleDisplayNameResult result)
    {
        Debug.Log("changed displayname: " +result.DisplayName);
    }
    private void OnPlayFabError(PlayFabError error)
    {
        Debug.Log(error.GenerateErrorReport());
    }