Dear All, I want to share a bug on TrailHead on the following challenge :
https://trailhead.salesforce.com/reports_dashboards/reports_dashboards_report_types

If you save your report in "My Personal Custom Report" folder instead of "Unfiled Public Reports", the error message state that your report is not "Matrix Format" even if it is. So, ensure you saved in the correct folder and your challenge will be OK.

Best Regards