Doing my happy dance!! Thanx for the explanation.This rule was put in place by the SMF admins. I think it was to prevent people from joining for the sole purpose of joining only to participate/vote in the challenges. But @MellonFriend is correct - it looks like you had reached to 50 post requirement in December, so you can join in this challenge if you would like!
Now for a REALLY stupid question. How to copy/paste??
Seriously computer illiterate.