A time off bank is always associated with an existing time off type. To create a bank:
Go under the Settings menu.
Click on the Time off > Types and banks tab.
Click on New time off type.
Add a name to your bank and check the Track in a time bank box to create an associated bank to every type of time off hours that must be recorded.
Click Save.
Once checked, you can start adding to and withdrawing from the bank. See this article to learn how.