If I understood your post, your network has, at least, three pieces of networking hardware: 2 AirPort Express base stations, and an Amplify wireless extender.
Yes, I have 2 Airport Express base station, an Amplify router and an Amplify wireless extender.
Would this Amplify device, be one of these products from this site? https://amplifi.com/
Yes. I have the Amplifi Instant System which has the router and one mesh point.
Which exact model(s) are your Express base stations? Are you only using them for AirPlay?
The Airport Express base stations are I believe 2nd generation. Yes, we only use them for AirPlay
What is the make & model of the device that is providing you with Internet access?
The modem is Motorola 24x8 Cable Modem, Model MB7621, DOCSIS 3.0.
This configuration was working fine until I reset the Amplifi router. Before that both Airport Expresses were doing AirPlay just fine. I reset because the internet was being wonky! One of the Airport Expresses is connected to the internet, but the Airplay is not working. I was able to connect it to an ethernet cable to set it up. The second one keeps giving me the error message.