Hi, @andreWi, and welcome to the Netlify community site.
NS1 is a DNS SaaS company. We use NS1 (https://ns1.com/) for DNS services at Netlify.
The reason for this error (“A DNS zone for this domain already exists on NS1”) is because some other company has already configured a DNS zone for this domain at NS1. Because it already exists, Netlify cannot create a new one.
We are not the only company that builds our services using NS1 as a component. For example, if I am not mistaken, Squarespace also uses NS1. If you configured this domain at Squarespace (or similar service) that will prevent us from doing the same.
There are two solutions for this.
The first solution for this will be to discover which other service you have used this domain with. Then contact that service to have them remove the configuration for this domain at NS1. That is the only solution which allows for using our Managed DNS for this domain name.
NS1 shouldn’t (and won’t) tell us who the other company is that already configured this domain with their service. This means you will need to determine which other services you have used for DNS service for this domain and contact that company to get this removed.
There is another solution. Instead of using our Managed DNS, you can also use manual configuration for the custom domain at Netlify.
You might also use manual configuration in the interim while you work with the other company to remove the DNS configuration they already have at NS1.
If there are other questions about this, please let us know.