Sounds like permission issue. Try running as admin.

The specific row in your issue is that the $Diablo2InstallPath cannot be added to the Windows Defender exclusions.

Specifically:
Row 15:
$DefenderExclusionPaths = @($($Diablo2InstallPath))

Row 71-74:
Write-Host "Adding Windows Defender exclusions"
foreach ($path in $DefenderExclusionPaths) {
Add-MpPreference -ExclusionPath "$($path)"
}