A time off hours bank is always associated with an existing time off type.
To create a bank, you must:
Go under the "Settings" menu
Click on the "Time off" tab
Click on "Add"
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!
Knowing that, see this article to know how adding hours to the bank you just started tracking.