An empty pipe element is not allowed in ForEach-Object

I cannot seem to find the exact issue with this code:

$backupCommand = @"
# Backup Outlook files
$outlookFolder = "C:\Users\$selectedUsername\AppData\Local\Microsoft\Outlook"
Write-Output "Outlook folder path: $outlookFolder"

# Verify the path exists
if (Test-Path -Path $outlookFolder) {
    Write-Output "Outlook folder exists."
} else {
    Write-Output "Outlook folder does not exist."
    exit
}

$backupFolder = "$outlookFolder\Backup"

# Create the backup folder if it doesn't exist
if (!(Test-Path -Path $backupFolder)) {
    New-Item -Path $backupFolder -ItemType Directory
}

# Get the list of .nst and .ost files in the Outlook folder
$files = Get-ChildItem -Path $outlookFolder -File | Where-Object { $_.Extension -eq ".nst" -or $_.Extension -eq ".ost" }

# Debugging output to verify $files
Write-Output "Files to be backed up:"
$files | ForEach-Object { Write-Output $_.Name }

# Check if $files is empty
if ($files.Count -eq 0) {
    Write-Output "No files found to back up."
    exit
}

# Process each file
$files | ForEach-Object {
    param ($file)
    Write-Output "Processing file: $($file.Name)"
    
    # Get the base name and extension
    $baseName = [System.IO.Path]::GetFileNameWithoutExtension($file.Name)
    $ext = $file.Extension
    
    # Construct the backup file name
    $backupFileName = "$baseName`_backup$ext"
    $counter = 1
    
    # Ensure the backup file name is unique
    while (Test-Path -Path "$backupFolder\$backupFileName") {
        $backupFileName = "$baseName`_backup$counter$ext"
        $counter++
    }
    
    # Move and rename the file
    Move-Item -Path $file.FullName -Destination "$backupFolder\$backupFileName"
    Write-Output "Moved and renamed $($file.Name) to $backupFileName"
}
Write-Output "Backup loop completed"
"@

# Execute the backup command on the remote machine using Invoke-Command
try {
    Invoke-Command -ComputerName $ComputerName -ScriptBlock { param($cmd) Invoke-Expression $cmd } -ArgumentList $backupCommand
    Write-Output "Backup process complete."
} catch {
    Write-Error "Failed to complete backup process: $_"
}

This is part of the code that fails Error output:

+  | ForEach-Object { Write-Output .Name }
+  ~
An empty pipe element is not allowed.
At line:34 char:2
+  | ForEach-Object {
+  ~
An empty pipe element is not allowed.
At line:49 char:11
+         ++
+           ~
Missing expression after unary operator '++'.
    + CategoryInfo          : ParserError: (:) [Invoke-Expression], ParseException
    + FullyQualifiedErrorId : EmptyPipeElement,Microsoft.PowerShell.Commands.InvokeExpressionCommand
    + PSComputerName        : V3000368
 
Backup process complete.

I already made all Write-Host to Write-Output, use ForEach-Object instead of foreach($file in $files) and multiple other modifications have been done; maybe someone knows what could be the issue?

Use a single-quoted here-string when assigning to $backupCommand:

$backupCommand = @'
... use single quotes, not double quotes...
'@

otherwise your code is being interpolated and variable names like $files in your code are being substituted with their values in the main script scope.

If they're not defined they get replaced with an empty string, so

$files | ForEach-Object { Write-Output $_.Name }

becomes

 | ForEach-Object { Write-Output $_.Name }

and if you try to execute that you get:

PS>  | foreach-object {}
ParserError:
Line |
   1 |   | foreach-object {}
     |   ~
     | An empty pipe element is not allowed.

You can easily confirm this by echoing the value of $backupCommand to the console with write-host $backupCommand.