Did you create it as a private challenge? If it is a private challenge, you will need to invite them by sending the link. If not, they will need to spell it exact in order to find it. The Challenge will only be visible if it has not started yet.
Here's message my coworker is getting:
This is what I get when I click on the link “You have to access to the invitation from your mobile device, thanks.”
When I try to access from my mobile device or the Go365 website challenges it says no results found.
718-983-7000 Ext 2123
1 John 4:8
When you invite them to a challenge, it sends a link. They will need to access the email from their Mobile Phone, or copy the link into their Mobile internet browser. Let me know if that does not work.