WoWInterface - Just Started Making Addons (XML Error)

Hello!

I recently retired from all of my jobs and wanted to do something to pass the time so I thought I would take up this hobby.

I started following various tutorials and such for making addons and when trying to make a XML frame in the Retail version of WoW, with no other addons loaded I receive this:

Code:

Interface/AddOns/OPAddon/OPAddon.xml:11 Unrecognized XML: Backdrop

This is the entire code:

Code:

<Ui xmlns="http://www.blizzard.com/wow/ui/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.blizzard.com/wow/ui/">



    <Frame name="OPAddon_TestFrame">

        <Size>

            <AbsDimension x="200" y="150"/>

        </Size>

        <Anchors>

            <Anchor point="CENTER"/>

        </Anchors>

      <Backdrop bgFile="Interface\DialogFrame\UI-DialogBox-Background" edgeFile="Interface\DialogFrame\UI-DialogBox-Border" tile="true">

          <BackgroundInsets>

              <AbsInset left="11" right="11" top="12" bottom="12"/>

          </BackgroundInsets>

      </Backdrop>

    </Frame>



</Ui>

I am quite unsure what I am doing wrong here. Is this XML outdated or?

Best Regards.