Connecting to px4 awaits for ever and doesn't connect

Hello, I am using MAVSDK and i am trying the example codes the point is that the codes fails to connect to my px4
i am using this script:

async def run():
drone = System()
await drone.connect(system_address=“serial:///dev/ttyACM1:115200”)
print(“Waiting for drone…”)
async for state in drone.core.connection_state():
if state.is_connected:
print(f"Drone discovered with UUID: {state.uuid}")

if name == “main”:
loop = asyncio.get_event_loop()

the point is that is enters a never ending loop and doesnt connect to my px4, although i doubled checked the address of the port /dev/ttyACM1:115200

I am trying to use it with ardusub firmware