Sorry for the issues you're having running a mesh test on your network. I'm sure that's quite frustrating to deal with. Let's see if we can get this worked out for you.
Often times when mesh tests fail, it's a connection to the the cloud services that is failing. I'm going to share the process for resetting that below.
Open the Google Home app.
Tap the WiFi icon.
Select Privacy settings.
Toggle Nest WiFi cloud services OFF and wait for 30 seconds, then back ON.
Once done, close the Home app and perform a full network restart to ensure that the new settings are in place. To do this, I would recommend doing a power cycle on your network.
Disconnect the power adapter and the Ethernet cable from the Nest WiFi router.
Disconnect the power adapter from the Nest WiFi points.
Disconnect the power adapter from the modem. Allow all devices to stay disconnected from the power for 5 minutes.
Connect the power back to the modem only and wait for it to fully back up. All lights should be lit up before moving to the next step.
After the modem is back online, connect the Ethernet cable back to Nest WiFi router's WAN port (globe symbol port).
Connect the power back to the Nest WiFi router and to the Nest WiFi points. Wait until they're all fully back on.
After running through those steps, run a new mesh test and see if it will complete for you. If you are still having issues, please come back and let me know and we'll look at other possible causes. Thanks.