Only the root user has the permission to write files in /var/lib/tomcat6/webapps/. So, if you try to do it using ec2-user, it will give an error, like the one in your case.
What you can do is:
a) Copy the files to /home/ec2-user/. You can use filezilla for that. Then SSH into your linux instance using putty. Change the user using sudo -s command.
b) Now copy file from /home/ec2-user to /var/lib/tomcat6/webapps/ using the following command:
cp -i RealEstateERP.war /var/lib/tomcat6/webapps/
You can also directly SSH into your instance, change root user using the same sudo -s command and then provide write permission to all users on /var/lib/tomcat6/webapps/ using the following command:
chmod 777 /var/lib/tomcat6/webapps/
Note: This second method is not recommended but it does work.