Why isn't my Skrill Mastercard valid in the Netherlands?

Go to http://www.apple.com/support/appleid/ an use the support link and see if the Apple support taff can answer that. We are fellow users on here so cannot speak for Apple.

The Skrill MasterCard is a prepaid debit card that you bought from a local store. No, that type of MasterCard is not acceptable. If you are a legal resident of the Netherlands you may only use a credit card issued by a bank in the Netherlands with a verifiable billing address in the Netherlands.

You can use the Skrill master card through Click and Buy.

in iTunes, go to your registered payment information, register click and buy as your default payment method, and use your Skrill card in click and buy.

Is it possible to use "CLick and Buy" in Estonia?

I don't have any credit card, only debit cards (I want to use skrill for payments).

Is it possible to bind it to my billing account? I need to pay icloud extra space. If it's not possible to pay with skrill, maybe there are some alternatives for icloud+iphoto?

On April 28, 2016, ClickandBuy will no longer process any transactions for the iTunes, iBooks, or App Store. You also won't be able to use ClickandBuy to pay for Apple Music subscriptions or in-app purchases.

For more information ☞ https://support.apple.com/en-us/HT206142