Thank you for the prompt reply, @kennedy442. I'll clear things up for you about marking bills as paid in QuickBooks Online (QBO). 

A credit memo is unavailable to use as an option to mark an open bill as paid. What you need is vendor credit instead. From the Bill Payment page, scroll down, and you'll see the Credits section.

If your vendor is also a customer, then refer to this article for the steps on how to proceed: Record a barter transaction.

Post a reply if you have other questions about open bills in QBO. I'll be sure to get back to you. Enjoy the rest of the day!