Can a site hosted with Squarespace be configured to work with IPv6?

It is best practice to have an ipv6 address, yet Squarespace does not have an ipv6 address.

Yes, Squarespace does not follow best practice, and no, there is no way to fix it besides to wait for them to update their systems.

It should only affect a very, very small amount of your users who are both (1) on ipv6-native networks and (2) with a malfunctioning NAT.