Must select at least one option in order for discount to be applied and not considered invalid
I can confirm this error with 4.5.4 / cividiscount 3.0. I had to individually select every price option in order for the discount to work at all. I think this is better as it's own issue so I'm referencing the comment from issue #35 (closed).
I believe this is a bug since cividiscount says:
If your form uses a price set, the discount will be applied to all options in the set by default. However, if you want the discount to be applied only to specific price options - select those options here.