If you downloaded the trial version of Avast! Pro, then the key will not work.
In fact, it will, when you add the key of Home version, your trial version will be 'converted' into the Home version. If you add the Professional version key, you`ll get full avast. The program identifies the key and become the correct version (Home or Pro) after you enter the password.
Better if you copy & paste from your email to the About dialog of the avast