Hi, @dellwatson. I double checked our NS1 account. The DNS zone isn’t under Netlify’s account.
This is the error we receive when we try to make a DNS zone under our account. Note, the error doesn’t say which other account created the blocking DNS zone, only that such a zone exists. No other details are given by the error.
My next recommendation would be to check with Name.com about a possible NS1 DNS zone still under their account. They might be able to find it and get it deleted.
Also, there are other companies that use NS1 as well. For example, once upon a time Squarespace used NS1 as well (and they likely still do). People having a custom domain on file at Squarespace was another common source of this error.
If no other services are using this custom domain, you might reach out to NS1 directly to see if they will tell you (the domain owner) who has created the DNS zone at NS1 under their account for this custom domain.
If there are other questions about this, please let us know.