When I try to add 1 or more to cart i get this message, mentioned in a lot of forums, but my nopcommerce is the 2019 version 4.20, it says The maximum quantity allowed for purchase is 0.
You will have to find your product in the Administration -> Catalog -> Products page and edit it. After that make sure that you've clicked the Basic/Advanced button on the top left corner of the page so it says "Advanced" otherwise you won't be able to see the setting. Then you can scroll down to the Inventory menu and expand it. You should be able to see the setting there.
I don't have that showing, very odd. I can't even add a picture here as it asks for a url!!! Here is part of the url in the browser /Admin/Product/Edit/1