Welcome fellow newbie. I assume since you're 20 days short of Platinum, then you are already a Gold Member. I had a friend go for a challenge just a couple of weeks ago. She was told that they do challenges only up to Gold. In order to get Platinum, you'll need to do the nights. Most of the airlines do the same. They typically only go to Mid-Tier Elite and never Top-Tier Elite.

It won't hurt to ask. Good Luck.