We were stuck in an error loop on Squarespace's side that I could not connect to my third party domain provider in this case Network Solutions. In our case this was caused by a former website that was nine years old and never used having been linked already to our domain name although it wasn't live.
We solved it by:
1) Having Network Solutions reset our account with them to default settings. (it was pointed to our old site that had never gone live). Squarespace requires it to be default settings as per the pdf they send you for this problem.
2) Logging into original squarespace account (figuring out that name.squarespace.com we only needed name as the ID not the rest) and resetting our password - we were in! We had two separate accounts. We subsequently deleted the first account once we unlinked everything and knew all was working.
3) under websites saw that this old website was linked to our domain name and clicked "unlink" and save
Then when we went back to squarespace to our new website account and followed their instructions for connecting to a third party we no longer got an error message and were provided the DNS records we sought. Then we could follow the directions provided by support on the website.