The ec2-user don't have permission to write files in /var/lib/tomcat6/webapps/. Only root user can do so. You have two ways here to do so:
1) Copy files to /home/ec2-user/ using filezilla. Now SSH into linux machine through putty. Change to root user using command sudo -s. Then copy file from /home/ec2-user to /var/lib/tomcat6/webapps/ using command cp -i RealEstateERP.war /var/lib/tomcat6/webapps/.
2) SSH into linux machine through putty.Change to root user using command sudo -s.Provide write permission to all users on /var/lib/tomcat6/webapps/ using command chmod 777 /var/lib/tomcat6/webapps/. Then copy files to directory directly from filezilla.
