Because you’ve factory reset it (which is not recommended) you need to consider it a new product and follow the process for a new device. Have you done that? If so, at what point in installation does it fail and what error message do you get? Just telling us “it will not connect” is not too helpful. 

 I’ve tried to add it as a new system.  I can get the arc to connect to the app - does the chime and I’ve pushed button on the back. It can see the wifi network but when i put in the password for the wifi it tells  me it can’t connect.     Try again!   All other devices will connect fine with the wifi 

On a small screen so finding links is hard, but if you go to Support here and search for "wifi settings" you can check that your wifi is in a compatible mode.

Thanks did you mean sonos support in the app? Or was there a link i cant see. 

Here’s a link to Sonos Support . You can also scroll to the bottom of this page and click on ‘Contact us’ if you prefer. 

Top of the forum page, Shop Learn Support i think the language portion may change for some places.

https://support.sonos.com/en-us/home

https://support.sonos.com/en-us/search?q=Wifi+settings

Thanks for your replies interestingly The Sonos chat bot also couldn’t detect what the problem was which turned out to be a simple setting with a WPA settings. I disabled WPA three and everything worked fine

Strange. As I understand https://support.sonos.com/article/supported-wifi-modes-and-security-standards-for-sonos-products the Arc should do WPA3. Good that it works for you and that you’ve mentioned your solution here.