To create a new expense reason first you need to activate the module. To do so you need to access your Back office, select Configuration - Your Modules - Data Analysis and enable the option Expenses.
Once the module is activated you will see in your Back office the segment Your expenses
Click over the green button + New Expense and a window will open to add the expense reason:
Add the new Expense and now you will be able to see this expense reason over your app to use it.