Error mapping home directory with a PowerShell script - powershell

Question

I'm using a script to create new users and everything is working fine. However, when I try to log in with a user created with this script, I get the following error:
A Problem has occurred and your network home directory is not available.
This may be because a network file server is offline.
You have been logged on with a temporary home drive (H:) which may be shared
with other users.
The script creates the user and applies the correct permissions as far as I can tell.
Here's the script:
# Import active directory module for running AD cmdlets
Import-Module activedirectory
#Store the data from ADUsers.csv in the $ADUsers variable
$ADUsers = Import-csv .\Create_Staff_Accounts.csv
#Loop through each row containing user details in the CSV file
foreach ($User in $ADUsers)
{
#Read user data from each field in each row and assign the data to a variable as below
$Username = $User.username
$Password = $User.password
$Firstname = $User.firstname
$Lastname = $User.lastname
$OU = $User.ou #This field refers to the OU the user account is to be created in
$group = $User.group
$title = $User.title
$start = $User.start
$Homedrive = "H:"
$UserRoot = "\\servername\st$\"
$HomeDirectory = $UserRoot + $Username
#Check to see if the user already exists in AD
if (Get-ADUser -F {SamAccountName -eq $Username})
{
#If user does exist, give a warning
Write-Warning "A user account with username $Username already exists in Active Directory."
}
else
{
#User does not exist then proceed to create the new user account
#Account will be created in the OU provided by the $OU variable read from the CSV file
New-ADUser `
-SamAccountName $Username `
-Name "$Firstname $Lastname" `
-UserPrincipalName "$($username)#domain.local" `
-GivenName $Firstname `
-Surname $Lastname `
-Enabled $True `
-DisplayName "$Firstname $Lastname" `
-Path $OU `
-AccountPassword (convertto-securestring $Password -AsPlainText -Force) `
-Homedrive $Homedrive `
-HomeDirectory $HomeDirectory `
-ScriptPath "logonscript.bat" `
-Description "Staff Account: $($title) from: $($start)"
#-ChangePasswordAtLogon $True `
#Now create the home folder and set modify permissions
Add-ADGroupMember -Identity $group -Members $Username
Add-ADGroupMember -Identity groupname -Members $Username
Add-ADGroupMember -Identity groupname -Members $Username
New-Item -ItemType Directory -Path "\\servername\st$\$($User.username)"
$path = Get-Item -Path "\\servername\st$\$($User.username)"
$acl = (Get-Item $path).GetAccessControl('Access')
$AR = New-Object System.Security.AccessControl.FileSystemAccessRule($Username, 'Modify', 'ContainerInherit,ObjectInherit', 'None', 'Allow')
$acl.SetAccessRule($AR)
Set-Acl -Path $Path -AclObject $acl
}
}
Thank you for your help and support.