3
The C# is heavily typed, that is, when assigning a value to a string, for example, the value should also be a string.
Try it this way:
MessageBox.Show("O jogador " + jogador_atual + " ganhou!");
reniciar();
jogo++;
lb_numero_jogo.Text = jogo.ToString();

First, what type of variable is "GAME" int? For what the error says is "It is not possible to make an implicit conversion from "int" to "string"
– user139461