I would assume Sonos is waiting for legal permission from Google for access. 

@Luís Filipe Guimarães Try changing your country to Spain in your Sonos profile. Google Assistant is supported on Sonos in Spain.

@GuitarSuperstar, let me thank You for your suggestion.

I can’t change the country to Spain or any other,  because the setup procedure demands me to activate “Location”. Therefore, I’m automatically identified as being in Portugal...

Try logging in to your Sonos account on the Sonos website on a web browser and change your location in your profile.

@GuitarSuperstar, Thank You for your help!

It really worked. The device now appears in Google Home and in a Division of my Home. Although, I can’t add it to the Groups I have in Google Home because the device is not visible to add.

It seems like this issue as going on for more than to years at least now. At this point Sonos seems like a nice paper weight. Seriously considering returning mine.

What issue?

Of not having support for Portuguese user 

As stated above this looks like it is depending on Google. Also explained above is a work around: change your country settings to Spain. Did this not work for you?

I have my account located in the UK.

I was able to add Sonos devices to Google Assistant.

Yet, when I request Google Assistant to play, for example, Spotify in one of the speakers linked to Sonos I get the reply.

“I am sorry that mode in not available on speaker XXX”

Any recommendation on how to address this?