I purchased and downloaded Lightroom 5.5. After installing it, I was prompted for the serial number. When I entered it an error message appeared saying it was an upgrade serial number and I need to enter my original purchase number. This was my original purchase, not an upgrade. Now what do I do?
It sounds like you bought an upgrade from Adobe. How much did you pay? The upgrade is about $80, and the non-upgrade is about $150. If you bought an upgrade version and had no previous version, you would see what you're seeing.
If that is the case and I were you, I'd call Adobe, and arrange for a refund and to order the correct version.