I had this same issue - but it turns out it was because I had marked this product as “out of stock” while I was testing the website (for obvious reasons). When I changed it to “in stock” add to cart worked.
This is SUPER confusing because the error thrown has nothing to do with the issue.
I would say double check that your product is available for purchase and try again. (Republish site in between)
And let’s pray together that WiX creates a better sandbox environment for testing out store customizations