The (intended) website is secure, and is protected with TLS. The specified website is misconfigured.
The Squarespace certificate associated with the webpage you were attempting to access is not trusted.
Some general Squarespace TLS troubleshooting info:
https://support.squarespace.com/hc/en-us/articles/360028492471-Troubleshooting-SSL
Using the local TLS troubleshooting tools, it’s the added www present in the URL you are specifying that’s causing the issues here. Remove that as it is unnecessary here, and the existing certificate will match to domain and the connection will work without issue and without warning.
Alternatively, ask Squarespace to allow users’ adding www with their TLS certificate used here.
For how to access a trust-blocked website (with a mismatched cert, or no cert, or some other issue) without the warning blockage, attempt the connection, then tap on the address bar, and change the https:// {rest of URL} to http:// {rest of URL}
Any data you receive or send related to that website (when accessed via http, without TLS) is trivial for anybody connected anywhere between your Mac and that website to monitor and log, and even to change, including the contact info section shown on that webpage, as well as whatever content changes a malicious intermediary might want to insert.
For the website admin:
For yourself:
macOS 15.6 is current. Please consider updating.
Caveat: matching (trusted) certificates just mean the connection is secure. Not that the website is at all trustworthy.
Gosh, and I just love TLS and certificates and website configuration or specification errors. Such fun.