MySQL problem with updating two tables

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I'm encountering an error while trying to subtract one from the 'product_quantity' field in the 'products' table when adding a new customer who has purchased a product. I have two tables named 'customers' and 'products'. I want the quantity of the product the customer purchased to decrease in the 'products' table. The logic seems correct to me, but I'm still getting an error. What could be the issue?