Hey there, tyler28.
Adding and setting up COGS as the default expense account isn't possible in QuickBooks Online since they have different Account Type and Detail Type.
The ledgers showing on the lists are all expenses, so you'll need to choose an Expense Account when setting up your vendor's profile. If you wish to add a new account, you can click the Add new button.
I'll be sharing with you this article: Adding Products and Services, Customers, and Vendors to Lists. This will provide more information on how to manage your vendor lists as well as a brief video to serve as your visual guide.
It'll be always my pleasure to help if you have any other vendor concerns. Just keep me posted in the comment section down below. Have a nice day!