From time to time my add to cart button leads me this error called "Parameter Missing or Invalid: Requited Parameter Missing or invalid". This error mostly occurs when I enter the site from a new tab or incognitio mode. Then after I refresh the page and try to add to cart again the problem doesn't not occur. It doesn't matter which product I try to add to cart. All of them produces the same error. This is a huge problem for the customers they won't be having that kind of patience.
Do you mind sharing your store URL (and password if necessary)? There may be an issue with the form pass information regarding the quantity or variant or something else. Has any customization been done?
I help brands bridge the gap between complicated code and clear communication.