What speaker(s) do you have? I don’t think the Eras support GA. 

Sonos one.
Two of them

In the Sonos app, go to Settings, Services and Voice, Add a voice assistant. 

Yea, tried that. I can only see Alexa and Voice assistant from Sonos. No option to select GA.

Are you in one of the Countries/Locations mentioned in this link:

https://support.sonos.com/en-us/article/google-assistant-availability-on-sonos

If you are, then ensure your online Sonos profile ‘country’ settings are also set correctly.

Yes. I’ve set USA as a country in settings. 

If you’ve just changed the country setting in your profile and it’s still not working - then reset the Sonos controller App (Setiings/App Preferences’) and on reopening connect it back to the ‘existing’ system and sign in… then see if you can see/add the Google Assistant to your speakers.

No, unfortunatelly, this didnt do the trick either.

It ‘perhaps’ might be a DNS issue, so you could try a different DNS on your router too, maybe use either the Google public servers 8.8.8.8 or 8.8.4.4 or Cloudflare server 1.1.1.1 and (temporarily) switch off any security on the mobile device, such as VPN client, or Firewall etc.

If a (final) router reboot doesn’t solve it, then you are perhaps best to submit a Sonos system diagnostic report from within the Sonos App and note it’s reference and then contact/chat with Sonos Support Staff via this LINK  and see what the Staff can suggest to resolve the matter.

Ok. Thank you for your help.

And it’s always possible that Google uses a different method than the data ‘reported’ to them by Sonos for the location of the account. You sidestepped the question above, by merely stating you set your Sonos location to the US, and didn’t say where you actually are. Google could be using any number of other location verification processes. 

Could be, but in that case Alexa wouldnt work either. I’m from Slovenia.

No, Alexa (Amazon) may use a totally different verification method. They’re two different companies, and unlikely to do things the same.