(From https://github.com/beeware/toga/discussions/3024)

The one thing that stands out in the code is that you're treating usuario_input and mostra_mensagen_te as functions - in mensage(), you should be using user_message = self.usuario_input.value.strip(), self.mostra_mensagem_te.value += f'Você: {user_message}\n', and self.usuario_input.value = ''.

In future, please include the complete error message when asking questions.