URL aliases problem when assigning aliases for custom pages I have created some content type page around 10 pages after that I started creating custom page like page-node-11.tpl.php. I want to assign node number 11 to my newly created page but when a assign node/11 it will give me error msg

The path node/11 is either invalid or you do not have access to it

How to overcome this error ?
You are doing it in reverse order. This order is generally followed.
Create a page.
Note down the node number (ex : 7) 
Then create a file corresponding to that like page-node-7.tpl.php
You should not assign node/11 as alias as these are considered as internal paths.
You can give anyother name which mostly suits the page contents.