This worked for me!!
1. Download Google Home app. Go to ‘Home’  and find your device. Click onto device and the ‘unlink Sonos’ for each device you have. 
2. Go back into your Sonos app and set up your devices all over again. 
Google assistant will also be part of the set up and Voila. 

• Fritz Box 5490 router w/ 1750 mesh repeater connected over ethernet
  
• SonosNet used for Sonos system with ZP as the "hub" connected over ethernet to the router
  
• Google Home Hub also in use and already connected to Sonos and Spotify accounts
  

Hello,

I also have issues with enabling GA on my Beam.

I have reached to Sonos, and waiting for a reply.

Here is my experience:

It is almost three week now that GA has stopped working on my Beam.

When I "OK Google"ed, it responded with "there was a bleach try again in a few seconds" or other similar error.
I tried rebooting the beam, rebooting my router, took of the power off for the whole house for a minute and turn it back on. 
Non of these changed anything.
I checked with the  status.sonos.com website and all systems were operational. 
Also now, while writing this, all systems are operational, and I am experiencing this issue.

Next, I unpaired Sonos from GA, remove the surround speakers, did a factory reset to the Beam, and re-added it to the Sonos system.
Now, when I try to pair it again with GA, weather I am using the Sonos app or the Google Home app, it always ends with "No devices found".
I also tried changing from WiFi to Ethernet cable and back, it is all the same.
Currently all three Living room speakers are connected using cables.

Here are some symptoms I have:
When Sonos and GA are paired, I can see my Sonos speakers on Google home app.
- When asking GA to play music on the Bathroom Play:1 - it returns "Ok playing bla bla", and it start playing as expected.
- When asking GA to play music on the Living Room (Beam+2Play:1s) - it returns "Ok playing bla bla" and then immediately it sais "sorry it looks like the Living Room isn't available right now"
- On the Google Home app, When I go to the Sonos TV Control Living Room, the status change from "Connecting" to "Not responding" and vise versa each second.

Using the Sonos app, everything is playing normal.

I solved the issue by cutting the power of my whole house at once and turn it back on again. Connect

Just an FYI that I ran into this issue with multiple Pixel phones running Android 10 recently when trying to add the Google Assistant to the Sonos Move.  An almost endless hang on either fetching the assistant to perform the integration or the assistant looking for devices. 

I ran the integration on an IOS device and it worked fine (after installing Google Assistant on IOS).

Hope this helps!

Just had this problem.

In my case, I had reset my sonos controller on my android device plus I had separated a pair with google assistant already set up. If you have used a pair you might have noticed one is a master and other a slave with the master only responding to google assistant. The one I am setting up was the slave.

To fix it 

I still had my sonos "skill" whatever it's called in my google home app so after going through the sonos setup I unlinked the sonos skill from google home, and added the speaker to a home and room, tried to setup google home in sonos and it worked.

I think because I no longer use the master but had reset sonos, Google home still had a memory of the master? And because only one works with assistant out of a pair it would not let me add another until I cleared it by relinking my sonos account to home again.

Hope that helps 

In case it helps anyone, I managed to resolve my similar issue of Sonos One’s  Google Assistant integration problems (after previously working fine for many months) where it would take many, many retries for Google Assistant to recognize my Sonos One speakers to integrate them in the first place and then sporadically having SOnos One’s Assistant respond with “there must have been a glitch” and “sorry, something went wrong” after integration. 

Problem was the enabling of IPv6 on my home network.  I’m guessing the Sonos One’s preferred IPv6 once getting an address but couldn’t reach out to the Internet or talk with my Google Home’s over it which caused the problems.  Disabling IPv6, everything started working fine again.

Unsure if this is on any roadmaps for being addressed. 

HtHs.