Hi @Elshan, welcome to the forum! This error means that either there is already an open change waiting to be completed. Or you are trying to make a change in your plan that isn't allowed. Without further data I can't tell which of the two cases it is. So, if you could fill out your forum profile and tell me what it is you are trying to change, I can help you further.

KPN is terrible. 20 GB is very small, and I can't change the plan. I have already spent more than 40 EUR to buy internet... I am very regret that I chose KPN.

How much do I need to pay to cancel my annual contract?

I've looked into it and found the problem, @Elshan. You started your contract this month. Your plan is changeable on a monthly basis. This means that you will need to sit out the first month, and then you can change the plan. Which, for you, is on september 11. 

In regards to cancelling, you are beyond the 14 day free annulment period. Which means that, for a cancellation, you would have to pay the entire cost of the annual contract at once.