I'm Level 10 and one of three objectives which I need to complete is to "Run 5000m with Frozen Scarlett". First, I thought that I needed to wear Frozen's Artifacts, so I did that and ran more than 5000m in Sky Summit, but the mission wasn't completed. Then, I thought I should do it at Frozen Shadows so I tried it again, but it's still uncompleted. I chose Scarlett Fox with all of this. I'm really confused with this situation. Could you let me know how to complete this objective?
You have to complete 5K meters in one run with Scarlett wearing her winter outfit (which you have to "buy", as purple wrote). However, that may be difficult in the frozen shadows scene, so you can also do it in the "sky summit" scene.
The "Frozen Artifacts" reward (that is, the earmuffs) apparently don't count for being "Frozen Scarlet." For that, you need to be wearing her secondary outfit (with the ice ax, goggles and white jacket), which costs 60 gems to acquire.