Hi Ramon, sorry you're having trouble with that.
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.
Next, these instructions might also help you to convert the software over from Lightroom CC to LR6:
If you still get the same error message regarding your serial number, 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.
Hope that helps!