There are few possible reasons for this error:

Reason: Site or (and) MID code is invalid.
Solution: Check if provided Site/MID codes are correct. These are hexadecimal numbers so they can contain only numbers (0-9) and letters (A-F). Instruct end user to copy/paste codes or automate this process from your application by using advanced protection interface. Using custom buttons with appropriate action string for sending Site/MID codes by email is also an option. 
Reason: Site/MID codes are generated by application protected with old V5 version of  PC Guard for Win32 or PC Guard for .NET.
Solution: Check V5 option in activation panel.
Reason: Wrong project is loaded.
Solution: Be sure that you have loaded correct project settings before trying to decode Site/MID codes and generate valid activation codes in activation panel.