That only works if you have set your nginx to serve /var/www/certbot for all of those domains. I mean, it can work but it's probably not the easiest of ways.
Have you tried just using
certbot certonly --nginx -d ambitx.io -d www.ambitx.io -d ws.ambitx.io -d rk.ambitx.io
instead?
If you want to use --webroot you'll have to do something like this: Certbot renew fails even when the challenge HTTP request is working - #22 by 9peppe (or once for all domains, if you don't want to serve them on http: Auto authorisation fails but manual authorisation using --debug-challenges works - #2 by 9peppe)