When installing the upgrade on a fresh system, if it does not find the required "old version" it will prompt for the old. It has been a while, but I seem to recall just entering my version 11 product ID and that worked.
I tried entering the V11 License Key but it didn't work. They seem to make it pretty difficult compared to most software these days - even Adobe just asked for my user ID and Password to download and install a fresh copy of Creative Cloud applications and they used to be one of the worst about re-installing. I doubt I still have a CD/DVD version of FileMaker around anymore now that we're in the download era.
If you registered your software with FileMaker, call customer support and they can provide you with a new down load link for an installer, but are you sure you didn't just try to run an updater for updating one version of 12 into another?