Try resetting the app and connecting to your system again. 

Check for an app update if you are on the new app and update to the latest version.

I reset the app, checked for updates (auto updates are ON) and confirmed there are no updates. The instructions state…

1. Go to the Settings menu.
2. Under My System, select your Era 100 or Era 300.
   • The Era 300s do not appear here as they are added to the Arc as surround speakers. If I navigate to Arc, under Products, I see Era 300 (Left Surround) & Era 300 (Right Surround).
3. Under Sound, select Trueplay, then Trueplay Tuning.
   • After selecting Arc, I see the Sound section with the following options (no Trueplay option)…
     • EQ
     • Surround Audio
     • Sub Audio
     • Height Audio
     • Volume Limit
     • Spatial Music (this is enabled)
   • If I further select either Era 300 speakers from the Products menu, there is no Sound section.
   • If I select any of the above Sound menu items, no Trueplay option.

I fully believe the Trueplay option is no longer available after connecting the Era 300 speakers to the Arc as surround speakers.

NOTE: I am on <1yr old android device.

You didn't state that you were using an Android phone in your initial post. I do not believe Trueplay is available for Android devices as stated here:

https://support.sonos.com/en-us/article/trueplay-compatible-devices-and-sonos-products

and here:

and here:

If I thought that mattered, I would have. From a development perspective, it shouldn’t matter. Also, it was definitely there when the Era 300s were not part of the surround set up.

Removing the IOS relevant information, the link states…”Android devices...can use the quick tuning function of Trueplay, if available. Quick tuning is currently supported by Era 100 and Era 300”.

So, the “if available” part must translate to, “We put Android devices second in priority to Crapple devices when developing the apps, and therefore are limited in functionality. We are going to advertise functionality that Android users won’t have so we can make our products seem better than they can truly function”.

Surely someone working at Sonos owns an android device and could test the functionality of the app and provide complete documentation. This is a disgrace to the Sonos name and all the incredible work the engineers are capable of achieving.

Good heavens. Over the years, there have been scores, if not hundreds of threads about why Sonos can’t set up TruePlay on the various Android devices, and their random microphones, each of which would need some sort of OS identification, and profiling. 

It does work, I removed the Era 300s from the surround setup and it worked as a pair. Why the limitation? Doesn't make sense to me. I'm going to use wife's iPhone next. UGH!

There’s a difference, if I’m understanding your post properly, between TruePlay run on a set of speakers, using the built in microphones in those speakers, such as on the Era 300s, and the TruePlay run on a iOS device, and not available on Android devices.

I wish they’d use a different ‘name’ for the two, it leads to confusion (like mine). Two different ‘sorts’ of TruePlay. And no, I have no idea why you can’t run TruePlay on an Arc, as it does have a built in speaker, but I’m assuming there is some reason.