Exactly the same setup, I have the same error. 
Before adding the Eras 300 I was able to use trueplay and tune my Beam gen2, but as soon as I added the surround, I can’t use the tuning anymore. 

I get “Something went wrong” when doing the position Trueplay and it just fails ¾ of the way into doing room Trueplay. Running an Arc + Era 300s as surrounds. I am really starting to regret buying the Eras as well as updating the app…

Exactly my issue, same set up Beam + 2 x 300s. Goes through the motions of playing test tones and then this very poor show from Sonos, it was the clunkiest of set ups too! My first experience of Sonos and I’m not impressed by the software at all.

Exactly the same as mine … i hope soon they will fix it.—

Please add me as a user with 2 ERA 300s (Surround) and an ARC with exactly the same problem!

Same here.  I have an arc and recently got 2 ERA 300s to use as surrounds, but I get the same error trying to set up True Play.  I was forced to upgrade the app just so I can add the surrounds. 

Update: need to retreat, i just updated to latest iOS app from today and it works now!
 

Same here. Very frustrating. Bought the Beam 2.gen on Monday and it is not working. I immediately get this error and no tone is played at all. I am wondering too if i can do anything to get it to work…

Have you updated the app and the speakers as well? 

Just the app. The Beam was updated on Monday immediately after unboxing, when i had the old version of the app though...

Try to update the beam again, a new version came out on Tuesday 

Same error for me with only the Arc (no other speakers). After reading forums, I did a reset of the soundbar, closed all other apps on my phone, still not working. 

Just FYI, TruePlay was broken in the beginning at launch of the new App. I even reported it to Sonos Tech Support who acknowledged the issue. Upon the first or second update TruePlay was working again. 

Something of note is that TruePlay seems to be more sensitive (or accurate) meaning that noise you may ignore as insignificant Trueplay will pickup thus disrupting the process. Low hum noise from freezers, and/or refrigerators. Noise from fish tank pumps, microwave oven, outside AC units, neighbor mowing grass the list can go on and on. Some noises are out of your control but it would behoove you to check those that you know and those that you may have previously deemed insignificant. 

I’m not at home so I can’t run Trueplay on my system to determine if it’s broken on my end. You might also check the microphone on your device to make sure the protective grill is free of debris. Of course you want to update all software as well.

I hope his helps!