"... because file is not modifiable" is the error message?
This error appears when your operating system prevents the application from making any changes to the file. It happens when the file or the folder containing it does not have the neccessary "write" permissions to permit the current user to make changes. I'd check the security settings on the file and it's enclosing folders to see the current write permissions, (In windows, you'd right click the file), unless you've recently moved this file to a new location. Either the settings have been changed or there are issues with your computer's user account settings.