A time off hours 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 on Save.
Once checked, you can start adding and withdrawing hours to it. Also see this article to know how adding hours to the bank you just started tracking.