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It depends exactly what you're trying to do, but I suspect you're trying to edit a footprint (package) that is already assigned to a device. If you look in library editor, every library has a number of packages and symbols (which are the schematic stuff), to create a device you attach it to a symbol and assign the pin numbers. Once a package is assigned and the pins are connected, you cannot add new pins or change pins on the footprint.
The easiest way to get around this is create a new package, either from scratch or copy the one you're trying to edit, and alter it to your hearts content. You can then assign it to whichever device you were wanting.