So were you trying to open Payables period without setting up Payables/Financials options? I wish you could you have mentioned that earlier. In Oracle things are dependant on each other. You cannot go to the next step without first completing the pre-requisites and hence all this issue. Please keep that in mind going forward.
About your quesiton on inventory organization, no it is not required for Payables unless Purchasing is enabled. Are you using Vision instance or PROD instance? In Vision instance Purchasing is marked as installed and you will be forced to enter an inventory organization. You will need to create a dummy one.
http://handsonerp.com/forum.html?func=view&id=2070&catid=2#2070