Unable to add configurablr product to cart

We're using 3.0.2. If a product is marked configurable and has product groups assigned to it on the Configuration tab, the “Add to Shopping Cart” button does not add the product (or the options) to the cart. Nothing at all seems to happen.

If we disable the Configurable checkbox on the Add-on tab, then the product can be added to the cart.

We have another store running 2.2.4 and the configurator works fine there.

Any ideas as to what might be wrong?