This issue affecting re-registration of Avast Free has been affecting users for several weeks. Avast have attempted one fix that seemed to fail, but yesterday announced another backend fix that should allow selection of Free instead of only Premium.
See this: Unable to reactivate Avast Free antivirus - #120 by prokopes_
If that does not work for you, please report on this thread: Unable to reactivate Avast Free antivirus