A whiteboard challenge is common during later-stage interviews. During the challenge, you'll typically get 20-45 minutes to show your thought process on how to solve a problem. First, the interviewer will briefly describe the problem you're trying to address, then open it up to you. Then, you'll lead a (short) discovery and design session while talking through and illustrating your process.
The most important thing is that you can talk through your process (rather than drawing on the whiteboard silently for 30 minutes, then showing your final solution at the end). It is not expected that you'll come up with an incredible, beautiful, high-fidelity solution in such a short amount of time. Focus more on explaining your process and exploration rather than spending lots of time on a final outcome.
You can think about the challenge in 4 steps: