Can you check your country within your Sonos profile by logging into your account on www.sonos.com.  Also check the country of your Amazon Alexa account.

This may be a wording issue in your post but you should be selecting ‘+ Add a Voice Assistant’ in the top section not ‘+ Add a Service’ in the lower section.

The section you’re referring to in the Sonos App is to add the Alexa assistant to compatible Sonos speakers - Looking at your online community profile you have Play:1/3/5 speakers and none of those are compatible to add an Alexa Assistant to them. You would need a compatible device like a Sonos One, for example. 

Despite the above, you can still use the echo device and the Alexa Assistant to control music playback on your older Sonos speakers, so the Sonos Skill will still work for you, if installed correctly in the Amazon Alexa App. I suggest aswell as checking your country profile, as suggested by @UKMedia, that you perhaps also research further "Amazon Alexa ‘enabled’ Groups" available in the Amazon service, as you may find their features quite helpful when voice controlling music playback on your Sonos speakers.