This device isn’t eligible for the requested build

Hi,

I am using iTunes to change the contents of my iPod, just this morning when I want to add and remove some files with my iPod, I meet an error message that tells me that my iPod could not be restored, I posted below the screen shot of the error.

iTunes

The iPhone  “       “ could not be restored. This device isn’t eligible for the requested build. 

As I read the error, I don't have any idea on the error. So, what I did is, to restart my computer and attached my iPod again, but same error message I encountered. I use the internet to look for similar problem and solution, nothing was posted.

Please share some thoughts on this error message, thanks.