I set up a Size group and customers can't add the item at all. What did I get wrong?

Almost always "Max picks" versus "Must pick". Must pick means the customer has to answer before the item goes in the basket; Max picks caps how many choices they may take from that group. A one-of-three sizes group wants Must pick ticked and Max picks 1. If the numbers do not add up the customer is refused at checkout in plain words — "Pick a Size for Chicken Biryani" when a required group was left blank, or "Too many Size choices for Chicken Biryani" when they exceeded the cap — and those rules are enforced on our side, not just in the browser, so a fiddled page cannot get around them either.