You have to create the vendors/suppliers in order to buy something.
How to add a vendor
Step 1: In “Purchases” you can add vendors.
Step 2: To add a vendor click “Add a vendor”.
Step 3: After entering the basic details like currency, country,state,address, city and postal code click “Save” option.
Step 4: You can “Edit” the vendor and also “Delete” the vendor.