It's possible that you don't have the latest version of Stuffit Expander (or whatever they call the freebie version now).

It's also possible that whoever stuffed the file didn't stuff it properly, or that somewhere along pottentially hundreds of transfers on LW, part of the file, during transfer became missing. If even a byte of data is missing from a stuffed file, Stuffit will be unable to expand it.

Can you attempt to download the file again from an alternate source if this is the case?