Scroll to the top of this screen. In the top right corner in Blue writing should be the word "Support"
Click on that.
You'll open a new page, on the top right there will be three options. The first of those three options will be "submit a request" - click there.
This should open the request page, select in the dropdown box "I have a question about League of Legends / Riot Games"
Fill in the boxes on this page. Under the "category" section, click on "Billing and Store" then it will change to a new menu and click on "RP Purchase Errors".
Hope that's clear ;)