First and Second gen Airport express Utility

I'm trying to connect to my Airport Expresses and the software I have isn't compatible

A 1st Generation AirPort Express is model number A1264. Look on the side of the device for the model number. It is hard to see, so you may need good lighting and/or reading glasses. If you see A1084 or A1088, this is not a 1st Gen Express, it is an older "b/g" wireless device that is incompatible with any of the newer 6.x versions of AirPort Utility.

Your Mac running Mojave has AirPort Utility 6.3.9 installed. The Yosemite Mac might be using AirPort Utility 6.3.5.

If you have the A1084 or A1088 version of the AirPort Express, you might be able to configure the device with a PC that is running the very old AirPort Utility 5.6.1 for Windows. To run the 5.6.x version of AirPort Utility on a Mac, it would need to be running an ancient operating system like Leopard or Snow Leopard.....11-12 years old now.

A 2nd Generation AirPort Express is model number A1392. You don't need to look for the model number on this device because this model has two Ethernet ports.

So to dig deeper, we need to know what model number(s) of the AirPort Express that you have. Please post back with that information.