When you say it won't work, could you please share more details on what's happening? Do you get any messages, and if so, what are they exactly?
These instructions might also help you out to convert the software:
i get the error message "we are unable to validate this serial"
First off, if you're upgrading, then make sure you are entering two serial numbers in the order requested – both the new one for Lightroom 6, as well as the previous one for your older version. Both keys are required to activate the upgrade license.
If you get the same message, then try the steps given here, which usually clear up this sort of issue:
This page may also help:
If those solutions don't work, then you may need to contact Adobe directly about it as we are a user-to-user forum here and don't have much power to help further:
via live online chat is usually fastest/easiest ("Still need help? Contact us"). Be sure to have your purchase information handy.