A DNS zone for this domain already exists on NS1, the DNS provider backing Netlify DNS

solve my problem now

The solution is right there in the screenshot your provided:

how to contact them help me

The link is right there in the screenshot as well, but you can go here: https://ns1.com/support

but how can contact them?
kindly guide me

Did you try going on the link? It mentions an email. Send an email to them saying, you need to clear the DNS zone for usa-genf20plus.com.

ok I just emailed them. how much takes time?

NS1 is not a Netlify service. We cannot speak for their support response time.

ok thank you for your support