You can set your Challenges so they're only available to users above a certain Learning Rank.
There are two ways to accomplish this: by editing an existing Challenge, while creating a new Challenge. This article will show you both methods.
By Editing a Challenge
Start by navigating to your Challenge's page. You can do that by hovering over Manage in the top navigation bar, and the selecting Manage Challenges.
Once you're on your Challenge's page, click the Manage dropdown at the top-right, and select Edit Challenge. This will take you to the Challenge Edit interface.
Use the Minimum # rank requirement field to specify the minimum Learn Rank users must have to submit projects to your Challenge.
Ranks start at 0 and are available up to 30.
While Creating a Challenge
Start by creating your Challenge. In the header bar, hover over Create, and select Create a Challenge. This will open a pop-up interface.
In the first step of the Challenge Creation interface, scroll down to the Eligibility Requirements section, at the bottom of the page
Use the Minimum # rank requirement field to specify the minimum Learn Rank users must have to submit projects to your Challenge.
Ranks start at 0 and are available up to 30.