looking at your error I suppose you have a scheme same as the above since your error is complaining about the source class MSH which does not exist within the XML document.

tSC1=target.SetValueAt(source.MSH.EncodingCharacters,"MSH:EncodingCharacters","set","")

do you have this class if not then that's where the problem is look at the bold

This will do exactly what you want any problems give us shout

ClassMethod ImpCSV(fileName As %String = "C:\filepath where csv file is")
{
	;;Refresh globals data at the begin of proecess
	;;globals hold the file being written to data and the count of lines each file has 
	k ^ufile,^ufile2,^counter,^counter2
	set Reader =##class(%Stream.FileCharacter).%New() 
	
	;;link to the file to read	 
	set status=Reader.LinkToFile(fileName)
		
   if $$$ISERR(status)
	{
		do $System.Status.DisplayError(status)
	}
	;;initialise the counter for the file lines to write to one file	 
	set ^counter =0
	set ^counter2 =0	
	set ^ufile=""
	set ^ufile2=""
    
    ;;the count for the lines read in
	set lineCount=0
	;;start process the lines
	while 'Reader.AtEnd
	{
		;;read the incoming file
		set line=Reader.ReadLine()
		;;increment the line count by one as you read the next line
		set lineCount=lineCount+1		
	
	    ;;piece the column to check
	    ;;one is for the first column and so on
	    ;;string at after the equal sign is the value to check for 
		if ($piece(line,",",1)="Religion")
		{
			;;increment count for this file
			 set ^counter=^counter+1
			  ;;the counter is at one to assign the file path to write to
			   if (^counter=1)
				{
					;;set the file name to write to the global
					set ^ufile=..writeFileOut("fileone",".csv",line,,^counter)
				}
				else
				{
					;;check the count has no reached the limit for the number of lines to write to
					if (^counter=51)
					{
						;;reset the count if limit reach
						set ^counter=1
					    ;;new file
					    set ^ufile =..writeFileOut("fileone",".csv",line,,^counter)
					    }
					    else
					    {
						    ;;use the current file
						    do ..writeFileOut(,,line,,^counter,"REG")
						 }
					}			
		  }
		  ;;second check for the next set of data if you have more than 
		  ;;two to check for copy the below and change accordingly to accommodate the next check
		  ;;same as above
		  if ($piece(line,",",1)="description")
		  {
			  ;;set attributes for the second file to write to
			   set ^counter2=^counter2+1
			   if (^counter2=1)
			   {
					set ^ufile2=..writeFileOut("fileTwo",".csv",line,,^counter2)
				}else
				{
					if (^counter2=51)
					{
						set ^counter2=1
						;;new file
						set ^ufile2= ..writeFileOut("fileTwo",".csv",line,,^counter2)
					}else
					{
						do ..writeFileOut(,,line,,^counter2,"DESC")
					}
				}				 			
	}
		  
}

 kill ^ufile,^ufile2
}

ClassMethod writeFileOut(filename, fileext As %String, line As %String, directory = "C:\directory to store files", linecount As %Integer, FileToUse)
{
	set oLF = ##class(%Library.File).%New()
	if (linecount=1)
	{
		
	    Set filenam=directory_filename_$i(^timmmy)_fileext
	    set oLF.Name=filenam
	    if (oLF.Open("wns"))
	    {
		    set linecount=linecount+1
		    do oLF.WriteLine(line)
		    do oLF.Close()
		    ;;w oLF.Name
		    quit oLF.Name	
	   }
	}else
	{
		if (FileToUse="DESC")
		{
		     if (^ufile2'="")
		     {
		         set oLF.Name=^ufile2
			}
		}elseif(FileToUse="REG")
		{
		     if (^ufile'="")
			 {
			     set oLF.Name=^ufile
			  }
		}
	    If (##class(%File).Exists(oLF.Name))
	     {
		     
		     Do oLF.Open("EWA")
		     if (linecount'=51)
		     {
			     set linecount=linecount+1
		         do oLF.WriteLine(line)
		         do oLF.Close()
		          quit oLF.Name
		     }
		 }
	}
}

Here is some code that will process your string container into lines of records that you want but you might want to change a few things to get this working with your records after converting your string container to a simple csv file you can then map it to your record mapper and process it any how.

Class TestEnvironment.Custom.GENERAL.ContainerString Extends (%Persistent, %Populate, %XML.Adaptor, %ZEN.DataModel.Adaptor)
{

Property Mystring As %String(MAXLEN = 32000, XMLPROJECTION = "CONTENT");

ClassMethod Import()
{
    // Create an instance of %XML.Reader
    Set reader = ##class(%XML.Reader).%New()

    // Begin processing of the file
    Set status = reader.OpenFile("C:\TEST\test.xml")
    If $$$ISERR(status) {do $System.Status.DisplayError(status)}

    // Associate a class name with the XML element name
    Do reader.Correlate("StringValue","TestEnvironment.Custom.GENERAL.ContainerString")
    
    // Read objects from xml file
    While (reader.Next(.object,.status)) {
	    
	   
        Write object.Mystring,!
        
//check the length of the string
         set lstr=$length(object.Mystring,",")
	    write "The length of incoming string is : "_lstr
	    if (lstr>28)
	    {
//divide by 28 for your processing
		    set div=lstr/28
		    if (div=2)
		    {
//assign your strings to something and write to file to map your records
			    set newstrObject=$piece(object.Mystring,",",1,28)
			    set secNewStrOJect=$piece(object.Mystring,",",29,56)
			 }
			 elseif (div=3)
		    {
			    set newstrObject=$piece(object.Mystring,",",1,28)
			    set secNewStrOJect=$piece(object.Mystring,",",29,56)
			     set thirdNewStrOJect=$piece(object.Mystring,",",57,84)
			     
			     write "this is the string cut1 :"_newstrObject,!
			      write "this is the string cut2 :"_secNewStrOJect,!
			       write "this is the string cut3 :"_thirdNewStrOJect,!
			 
		 }}
    }
    
    // If error found during processing, show it
    If $$$ISERR(status) {do $System.Status.DisplayError(status)}
}


}

hope this helps