help!!!! activation

I got as a gift a macbook pro with a previous version of Desktop parallel 11.
Today I decided to buy the upgrade to version 12. I will insert to activate the code you have sent me after payment. then he asks what the previous version .... I do not know what ... and then not turned me anything !!!
I already have an account where you are registered but can not enter the new code purchased because you tell me which is registered by other parties .............. what should I do ????? ?
if you need buy the full version but you have to make me pay only the difference !!!!!
Help me please.
Laura