Usually, this occurs when Product Discount has a discount applied to a product, and the product's variant was manually modified, a CSV import was done on the product, or the product was duplicated. Doing any of these things will cause the variant ID to change, which will remove important metadata on the variant which is required for Product Discount to function. At this point, you will need to change the product's price manually to its regular price. After that, you'll be able to use this product in a discount group again.
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