I read your sendMessage.py program a second time, and guess it is doing the following three things: (1) setup the nRF24 module, and print what has been setup, (2) Send the message "Hi" only cone, (3) Change to listener mode and listen for only 2 seconds, and print "time out" if not receiving acknowledgement from Arduino?

Now I think we can do the following to troubleshoot: (a) Instead of sending message just once, we can repeatedly send the same message every 4 seconds, and see if Arduino can get the message, (b) Instead of setting only two seconds time out, of course we can set longer, say, two minutes time out, so to give more chances for Arduino to respond.