Not sure what to do.
Suggest that you think about how the AirPort Express will connect to your network. If it will connect using a wireless connection.....then that is the way that you would want to set things up.....with a wireless connection.....not Ethernet.
Why? If you set up the Express using an Ethernet connection, then it will assume that this is the way that it will connect to the network, which is not the way that it will actually connect to the network, so even if it appears that the setup might work initially, the Express still won't be able to connect to the network correctly.
So, we'll need for you to clarify "how" the Express(es) will connect to your Eero network. Will they connect using wireless....or will they connect using a permanent wired Ethernet connection? The setup for wireless and wired is completely different, so trying to choose to "Add" the Express with an Ethernet connection won't work.
In other words, you need to go with the setup that the AirPort Utility setup "helper" or "wizard" suggests, not try to change settings and set things up manually.
Next item.......the IPv6 setting on your Mac may not be set to the correct Link Local Only setting for IPv6.....and the AirPorts use IPv6 for their communication on the network......so that might have a lot to do with the error(s) that you are seeing.
Finally, when all else fails when you are trying to use a Mac for the setup of the Express, sometimes an iPhone or iPad will do the trick. Why? The iOS version of AirPort Utility has fewer bugs than the Mac version.......which often surprises users.
