"This action cannot be performed because this file is not modifiable."
This is an error message that you get when the Operating System is prohibiting changes to your file. You have it either set to "read only" or located in a directory where you do not have the needed account permissions to modify the file.
I'm not sure. The file has Read & Write priviledges. I thought it even might have something to do with opening it from an alias on the desktop. However, I quit FileMaker Pro 11 30 day version then reopened it and clicked the alias. The file then started accepting changes again.