EntityType has no key defined. Define the key for this EntityType

Could someone help with this error for the class below:

Error: GEW.Models.Repository.Product: : EntityType 'Product' has no key defined. Define the key for this EntityType.

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Product.cs
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namespace GEW.Models {

    public class Product {
        public int pkProductID { get; set; }
        public string Description { get; set; }
        public decimal Cost { get; set; }
    }
}