In trailhead : Customize an Org to Support a New Business Unit > Automate Your Business Process

The steps are: 

create a new billing support case to test the assignment rule.

1. Click the App Launcher and select Service.
2. Click the Cases tab.
3. Click New.
4. Select Billing and click Next.
5. Complete the inquiry case details: Other details, I can find and fill. However, for "Assign using active assignment rule | Select" - I am not able to find the option. I even tried the note: below however it still does not work.

1. Note: If "Assign using active assignment rule" is not appearing when creating a new case, return to the Case Assignment Rule menu in Setup. Edit the Standard case assignment and uncheck the active checkbox and save. After saving, check the active checkbox again and save. This toggle will ensure "Assign using active assignment rule" appears when creating a new case.
2. Click Save.