If you buy an Upgrade you will need to have a previous version of Lightroom ... a Full version doesn't need a previous serial number.
Did you buy and "Upgrade" or "Full" version?
If you wanted full but bought an upgrade you will need to contact Adobe customer support.
You must have an Upgrade serial number that requires a serial number from an earlier version at full price as well.
I suggest you go back to where you purchased it and sort it out with them....