Confusing error message

    Hopefully this is the correct forum for this post.  I have several workplace computers I have been tasked with updating.  I am using Norton Internet Security Small Business Edition.  The package says it allows me up to 10 PC's worth of protection.  I have renewed 7 Pc's so far that serve as our cash registers.  The last three PC's I have tried to update however (which are not POS registers if that's somehow relevant) Have given me the following Error message:

 

    Invalid Product Key

    Your current product is Norton Internet Security. The Product key you entered is for Norton Internet Security.

 

    Download Norton Internet Security

   Type in a different Product Key

 

This confuses me. If my current product ive been using is Norton Internet Security, why is it wrong to use a Norton Internet Security product key?   I only have a few minutes at a time to access these computers before I hold up business and start annoying my bosses, so I am hesitant to uninstall, reinstall, and/or just generally tinker with anything without first seeing if that is indeed the problem. 

 

Any help would be much appreciated.