Just Started Making Addons (XML Error)

[OP3]

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:

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

This is the entire 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 Is this XML outdated or?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.