How i get challenges/missionen?

Didnt find anyone. In the forum some ask to switch challenges liek an everest or something… but i dindt found something were i can join. Some tell me on the start screen should be something to join, but i didnt found any. iam lvl 12 maybe i have to get higher level?

To select the challenge, stop riding. Click on the menu and then you select the challenge on this screen. Click on the icon circled in red.