Commented Unassigned: Script Fails right after Cache Account [21106]

I checked the XML file and it contains webapplication config

Exception : System.Management.Automation.RuntimeException: Method
invocation failed because [System.String] doesn't
contain a method named 'SelectSingleNode'.
at System.Management.Automation.ExceptionHandlingOps
.CheckActionPreference(FunctionContext funcContext,
Exception exception)
at System.Management.Automation.Interpreter.ActionCa
llInstruction`2.Run(InterpretedFrame frame)
at System.Management.Automation.Interpreter.EnterTry
CatchFinallyInstruction.Run(InterpretedFrame frame)
at System.Management.Automation.Interpreter.EnterTry
CatchFinallyInstruction.Run(InterpretedFrame frame)
TargetObject :
CategoryInfo : InvalidOperation: (:) [], RuntimeException
FullyQualifiedErrorId : MethodNotFound
ErrorDetails :
InvocationInfo : System.Management.Automation.InvocationInfo
ScriptStackTrace : at CreateWebApp, C:\Users\srvspsetup\Desktop\AutoInstal
ler\SP\AutoSPInstaller\AutoSPInstallerFunctions.ps1:
line 2632
at CreateWebApplications, C:\Users\srvspsetup\Desktop\A
utoInstaller\SP\AutoSPInstaller\AutoSPInstallerFunction
s.ps1: line 2497
at Setup-Farm, C:\Users\srvspsetup\Desktop\AutoInstalle
r\SP\AutoSPInstaller\AutoSPInstallerMain.ps1: line 204
at <ScriptBlock>, C:\Users\srvspsetup\Desktop\AutoInsta
ller\SP\AutoSPInstaller\AutoSPInstallerMain.ps1: line
397
at <ScriptBlock>, <No file>: line 1
PipelineIterationInfo : {}
PSMessageDetails :
Comments: ** Comment from web user: IvanJosipovic **

Without looking at your xml it looks like you dont have any site collections in the webapp.

Line 2632 will fail if there
if ($webApp.SiteCollections.SelectSingleNode("SiteCollection")) # Only go through these steps if we actually have a site collection to create

The code will fail if there are no site collections ie:
<SiteCollections>
</SiteCollections>

Can you post your config (with cdkey and passwords removed) to pastebin?

Thanks,
Ivan