commerce/cart/updateCart throws page not found Adding products to the cart throws a page not found (404) error that has this header;

Page not found.
↵
Caused by: Invalid Route – yii\base\InvalidRouteException
Unable to resolve the request: commerce/cart/updateCart
in /Users/a/Sites/ae-dev/vendor/yiisoft/yii2/base/Controller.php at line 128


Very strange. Anyone encounter this issue?
Based on your tag, I assume you're using Commerce 2.

Controller actions in Craft 3 use kebab-case, now, so your action should be commerce/cart/update-cart.