Hi Jane073,
Welcome to the Community. It's essential to correctly record your transactions in order to keep your books up to date. QuickBooks Online is a flexible program that simplifies the way you manage your books. I'd be glad point you in the right direction to write off bad debt.
If there are no remaining credits to apply to the invoice, this could mean that a payment or a credit memo is attached to the invoice. You can verify if the invoice is still unpaid or attached to another credit memo. I encourage you to click on Bank transactions on the left navigation menu > select All Sales > find your transaction.
I'd encourage to review this article on how write off bad debt in QuickBooks Online.
Let me know if you have any questions, I'll be happy to help.