It’s firmware could be too out of date to add via wireless.  Temporarily wire it to your router via Ethernet, then try to add.  Once it is added, allow it to update, then deploy wirelessly.  

it is connect via Ethernet

I would contact Sonos support.  There have been cases where the firmware is so old, they have to force an update from support. 

Is it perhaps an early 1st generation Sonos Connect Amp (manufactured prior to March 2015, I think?) and you’re trying to set it up using the S2 App? If so, you will need to use the S1 Sonos App instead. Also ensure your mobile controller device meets these system requirements:

https://support.sonos.com/en-us/article/sonos-app-requirements

that was the issue.