1) Check if you have this issue with the default theme: go to Appearance > Themes > install and activate Storefront theme. Test checkout again. Do you have this error when the Storefront theme is enabled?
2) If you have the same issue even if the default theme is enabled check the list of plugins that you use. Disable third-party plugins and test checkout again. If you still have the problem after that then contact WooCommerce plugin support. In case everything works fine without third-party plugins then enable plugin one by one and test checkout after every enabled plugin.
3) If you have the problem ONLY when the XStore theme is enabled and everything is fine when you switch to default theme then provide Dashboard and FTP access to your site.
I contacted WooCommerce and they suggested it was an account authorization issue and suggested disconnecting from Square (under integration tab on WooCommerce Settings in WordPress Dashboard) and then reconnecting to Square.