I am having an issue with this challenge as well, I am doing everything it asks and don't see what I'm doing wrong.  The instructions are:
Dataset: DTC Opportunity
Add Group: Close Date (Year-Month)
Add Group: Forecast Category
Change Measure: Count of Rows to Sum of #
Filter by: Closed Equals False
Chart Type: Timeline
Lens Name: Sales Pipeline Overview

I double checked I am saving to My Exploration and copy and pasted the title exactly from the instructions.  HELP!