From the Navigation Pane
Expand                   Self Service
Expand                   Setup
Double click on       Process Definitions Linking
Click on                  the Process Definition in question

Ensure your Leave Definition workflow setup as below:

From the Navigation Pane
Expand                   Company Management
Expand                   Leave
Double click on       Definitions
Double click on       the Definition in question

• Either the Leave Definition is set as Inactive 
• The Leave Definition is set set to Hide for ESS as below: