As noted by user "MITTS," the "Expense Voucher" case type is built from scratch. So, any given exercise assumes the previous exercises were completed. The exercise "Define Stages for Expense Voucher Case Type" assumes the previous exercise "Using the Application Wizard - Application Express" was completed.
If you have not completed the previous exercise, please do so. If you have completed the previous exercises, please let us know as something else would be causing the issue.