4/13 Update: !Challenges are possibly broken for some players!
The Coalition talk about the issue four minutes into their recent stream:https://mixer.com/GearsofWar?vod=97256907Please feel free to down vote this solution if you would like an alert for when I find out this is working again, I won't take it personally.
For those that don't know , TA guide editors can choose to alert down voters that they've updated their guide. The next time I update this guide will be to let people know that it's working again, I wont' spam you.
If you had reached re-up 10 or higher before the Challenge went live, it should have popped upon you loading the game and downloading your stats from the server.
If you are lower but don't see any percentage progress tracking, that's fine as that's how it's meant to be.
As stated in the video above, if you're re-up 8 it won't show 80% on the tracker.
Please consult these guides if you're looking for ways to level up fast: