Hi, dimendia
The “Exclude categories” setting refers to product categories that the coupon will not be applied to, or that cannot be in the cart in order for the “Fixed cart discount” to be applied.
If your coupon is a "Fixed cart coupon", then it will not be applied when you have a product from an excluded category in the cart. If it is any other kind of coupon, the “Exclude categories” setting will be ignored and that's what's happening here.