To integrate your Squarespace store with Selfnamed, you'll need a Business or Commerce plan (either Basic Commerce or Advanced Commerce). These are Squarespace's e-commerce plans that allow you to sell products and accept payments.
The Personal plan won't work for the integration, as it doesn't include the full e-commerce functionality needed to sync orders with Selfnamed.
If you're unsure which plan you currently have, you can check by logging into your Squarespace account and going to Settings > Billing & Account.
If you need to upgrade, you can do so directly from your Squarespace dashboard.
Once you're on a Business or Commerce plan, you'll be able to connect your store to Selfnamed and start selling your custom products!