I think you are using Vision Demo database. You are required to provide inventory organization to AP setup if Purchasing is in use. That is why I recommend that you use fresh/production install in order to learn Financials in a step by step manner.
The only option you have is to create a dummy inventory organization. Log in as one of the Inventory responsibility and create an inventory organization. Enter any account in Accounting Information Section. Also fill in values in Inventory Information Section. Assign this inventory to AP setup.