Contact phone support and reference that diagnostic. Meanwhile, avoid Factory Reset without further consult because it rarely cures this sort of issue and it destroys diagnostic data.

Reboot is OK.

On a wild speculation, try raising the Volume a bit before starting Trueplay. I'm not suggesting that you “peal the paint" or “kill flying insects”, just a little.

I called support with reference number (sounded like support from India) and they reviewed the diagnostics and found no apparent errors and decided it’s probably just because the speakers connected to the Sonos Amp aren’t Sonos brand. I’m pretty certain people have used trueplay on other brands as long as it was in the Sonos amp.

First thing I tried was raising the volume, but it seemed as though no volume control was available - like trueplay has a set level for the test? I tried moving iPhone mic closer to the front and back as well but still same error.

Only Sonos by Sonance architectural speakers will give you Trueplay: