Thanks for following up with us, @Ffraga. 

I've noticed in the past when we see this error message, it's usually because users have the bills set as "Schedule Online Payment" in the Pay Bills screen. If this is the case for you, let's set the payment method to Check. Then, try applying the discounts again. 

For more information about handling discounts in QuickBooks Desktop, check out Pay Bills in QuickBooks.

Please let me know if you run into any issues along the way. I'm happy to help. Take care!