Hello friend I also had that problem , but since I use it so it works try with the code below. if you do not have instantiated page use only page.update().

snackbar=ft.SnackBar(
    content=ft.Text(
        value="Login Ok",
        bgcolor="white",
    ),
    bgcolor="green",
    action="ok",
    duration=3000
)
self.page.overlay(snackbar)
snackbar.open=True
self.page.update()