<WebpartName>: The file is not checked out. You must first check out this document before making changes.
Scenario with Solution
While trying to add webpart to the page , i received the above error. After some minutes i got the culprit .
1) Even on clicking "Edit Page" my file was not checking out , so i had to check out my webform using sharePoint designer. This solved my intial issue of adding web part .
2) Now question was , Why the "Edit Page" is not chekcing out my page , well it was no brainer . Publishing feature in my Site Feature was not active.