Where do you live? Have you set that location on your Sonos account at www.Sonos.com? Sonos bases what they provide to you in their controller based on your location. If you live in an area where Google Assistant is available, it should be showing up for you. 

Have you contacted Sonos directly to check with them?

I live in Portugal.

According to the Sonos FAQ (also linked in my previous post), Google has not yet authorized the use of Google Assistant on the Sonos system for Portugal. 

You may want to contact Google and ask them why. Sonos does require authorization before they can offer a service.