Hi Anshul,

use after update BR on approval table and past this code in advance section.

var gr = new GlideRecord(current.source_table); 
gr.addQuery("sys_id", current.document_id); 
gr.query();       
 if (gr.next()) {             
gr.comments = current.comments.getJournalEntry(1);                
gr.update();    
 }

My Solution is helpful and accepted.