If you see this message when you try to create a Square Online site, it means you already have an account on Weebly associated with the same email address as your Square account:

Early access for new users only - While we work to make our Square inventory sync feature available to existing Weebly users, you can update your website by logging in here.

In a separate browser (or you can use Chrome incognito, Firefox private window, etc.), go to weebly.com and try logging in with the email address associated with your Square account. Don't use the "Log in With Square" option; the point here is to use the same email address to log into your legacy Weebly account. If it tells you that your password is wrong, choose the reset-password option. This will reset the password on your old Weebly account, at which point you can then log in.

Once you have logged into Weebly using the old account, change the email address on that account. Click your name in the upper-right corner of the main Weebly dashboard, and from there it's pretty straightforward. With the new email address in place on your old account, you can now go back to Square and create a Square Online site.

After you have changed the email address of your old Weebly account, you might want to just delete the whole thing. I don't have a legacy Weebly account handy to tell you the exact steps to do this, but it was easy to find. On the account details screen, there's a tab called "My Data" or something like that, and on that tab is an option to delete the account.

I hope this helps someone! I was super confused when I saw this, especially since clicking the "logging in here" link just takes you back to the Square dashboard. The key is to use a browser where you don't have Square cookies. Now that Weebly is owned by Square, there are all sorts of assumptions about the two being linked.