Special Promotions and Special Promotions Pro extensions require validation by category for associated simple products.
It means that when you create a new rule, using any of our promotion extensions with condition based on "category", it is necessary to assign associated simple products to the same category(es) as their parent configurable products.
So, open the Product Information page > Categories and check if the configurable product is assigned to a certain category.
Then open Product Information page > Associated Products > Edit simple product, and repeat the same action, as shown on the screenshot above for the simple product. In other words, you need to assign the simple product to the same category.
This validation helps you to avoid any possible issues during the checkout and payment steps and calculate the discount amount correctly.