Let's make sure you'll be able to apply a vendor credit to the same vendor, kennedy442.

Ideally, you can apply the credit to an unpaid balance of your bill. You'll be able to see the credit once you'll make a payment and click the Unapplied Payment link to double-check if you can apply the amount to the bill. I've attached a screenshot to make sure that we're on the same page.

However, if you're still unable to apply for the credit, I suggest you delete and recreate the bill. To delete it, follow the steps provided below: 

1. Go to Expenses.
2. On the Expenses tab, locate the bill to delete.
3. In the Action column, select Delete from the drop-down menu.

In addition, QuickBooks automatically downloads transactions and tries to look for a match with transactions you've entered. Please go through this article for your guide: Categorize And Match Online Bank Transactions In QuickBooks Online. 

Let me know if you have any concerns about applying a credit to a bill or if you're referring to something else. Always remember, we're here to make sure everything is taken care of. Stay safe!