Hi, @mjsorribas. The error means a DNS zone already exists at NS1 and because of this we cannot create one ourselves. Many other services use NS1 -for example, last I checked Squarespace used them as well.
The error doesn’t tell us what other account created the DNS zone at NS1. You will need to look for places you entered this domain with other services and remove it there. Similarly, you might try contacting NS1 directly to ask them what account created the DNS zone.
In the meantime, you can also use the external DNS instructions above to connect the domain to Netlify. This doesn’t rely on NS1 and can be used while you work on resolving that issue.
If there are other questions, please let us know.