Using Powershell and ReportService2010.asmx to get datasource details how to find ds path

I am using powershell and the ReportService2010.asmx endpoint to retrieve list of SSRS Reports and their dataSources from PBI Reportserver.  For background, I am calling these functions created by  written by Jana Sattainathan. ( I am rather new to powershell)
I am able to pull reports, and for each report I can get a datasource REFerence.  Bu now I am trying to call GetDataSourceContents so I can get the connectionString and other details.  From the datasource object I have access to datasource reference and name,  report path and name , But I think I need the path to the datasource itself. (Code below )
Can anyone help me figure out how to construct this value?  OR,  The GetDataSourceContents I am callin may not be the correct one?  It does not show up in my "commands" window in the ISE , but it pops up when putting a . after the $reportproxy I am using which is https://MyServer.com/ReportServer/ReportService2010.asmx

$Reports = Get-SSRSReport -ReportServerUri $PBIRSReportServerUri -ReportFolderPath $StartFolder

foreach ($Report in $Reports){
    Write-Host $Report.Name $Report.Path $Report.TypeName 

    $DSS = Get-SSRSReportDataSource  -ReportServerUri $PBIRSReportServerUri -ReportFolderPath "/AP Administrators"  

    foreach ($DS in $DSS){
        Write-Host   $DS.ReportName $DS.ReportPath $DS.DataSourceName $DS.ReportPath  $DS.ReportName
        $validObject = $reportproxy.GetDataSourceContents($DS.ReportPath) <---What should this value be?
        Write-Host $validObject.ConnectString  -ForegroundColor Cyan
    } 
}

Error Message: Exception calling "GetDataSourceContents" with "1" argument(s): "The operation you are attempting on item '/AP Administrators/Server Administration/SQL Datafeed History' is not allowed for this item type
Here is a sample url to a datasource on the server , but I don't think it helps
https://MyServer/Reports/manage/catalogitem/datasources/folder/subfolder/reportname?_pk=sdp2ph00