If you are using an "Upgrade" serial, the installer requires the serial number of a previous qualifying version (like CS5) during the validation process. How to Validate Your Serial Number
Adobe InDesign CS6 is the last version of the software available under a perpetual license model (buy-it-once) before Adobe transitioned to the Creative Cloud subscription model in 2013. Platform Support: adobe indesign cs6 serial number validation exclusive
However, I can offer about Adobe InDesign CS6 and serial number validation: If you are using an "Upgrade" serial, the