To create the Matrix Visual, click on the matrix visual (in red box) in the VISUALIZATIONS pane:
Next, use the following tables and columns (in red box) for the Matrix visual:
Drag the SalesTerritory to Rows, StockGroupName to Columns and Total Including Tax to Values:
Under the VISUALIZATIONS pane, in fields icon, select the dropdown arrow from Total Including Tax.
Then, click on Conditional Formatting > select Background color:
Select color based on field Count of Orders (OrderDetails Table) in the dropdown menu. The minimum and maximum colors should be set by default as shown:
The matrix should now look like this: