I've been using File_Girl's version (without AVG) but just installed the original Pirate Edition. I'm seeing the same error.

After installing, LW used to download AVG's latest definitions. Which the Pirate Edition didn't do. An error message tells me that the virus scanner is still downloading it's first set of virus definitions.
Maybe AVG is broken in PE.

Did AVG ever work in your PE?

Might be easier for you to use your own AV to scan files. If it's annoying you to see the error, you can uncheck the scan option (tools > options > transfers > uncheck 'scan files I download for viruses' > ok) then you shouldn't see the 'unable to scan' message.