Appending data to an XML file

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I have been working on an xml structure and Im having issues adding to a specifc part of the XML file and need help. My goal is to add new category under the Categories node. below is my current code.

$Path = "C:\Users\shuppz\Desktop\XML\Helpbox.xml"

$xmlDoc = [System.Xml.XmlDocument](Get-Content $Path)
		
#Create New Category
$newCat = $xmlDoc.AdminLauncher.Categories.AppendChild($xmlDoc.CreateElement("Category"))
		
#Set Attributes
$newCat.SetAttribute("Selected", "False")
$newCat.SetAttribute("Name", $Name)

#Save File
$xmlDoc.Save($Path)

My XML:

<?xml version="1.0"?>
<!--XML For Admin Launcher-->
<AdminLauncher>
  <Categories />
  <Task Name="GPMC" Category="Tools">
    <Path>C:\Windows</Path>
    <Arg>/offerra</Arg>
  </Task>
</AdminLauncher>

When I run it I get the below errors:

Method invocation failed because [System.String] doesn't contain a method named 'AppendChild'.
At C:\Users\shuppz\AppData\Local\Temp\5564e900-f0cf-4878-8c39-9fc3aff6ed8e.ps1:6 char:55
+ $newCat = $xmlDoc.AdminLauncher.Categories.AppendChild <<<< ($xmlDoc.CreateElement("Category"))
    + CategoryInfo          : InvalidOperation: (AppendChild:String) [], RuntimeException
    + FullyQualifiedErrorId : MethodNotFound
 
You cannot call a method on a null-valued expression.
At C:\Users\shuppz\AppData\Local\Temp\5564e900-f0cf-4878-8c39-9fc3aff6ed8e.ps1:9 char:21
+ $newCat.SetAttribute <<<< ("Selected", "False")
    + CategoryInfo          : InvalidOperation: (SetAttribute:String) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull
 
You cannot call a method on a null-valued expression.
At C:\Users\shuppz\AppData\Local\Temp\5564e900-f0cf-4878-8c39-9fc3aff6ed8e.ps1:10 char:21
+ $newCat.SetAttribute <<<< ("Name", $Name)
    + CategoryInfo          : InvalidOperation: (SetAttribute:String) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull