Yes, you'll have to make a  credit memo to write off an unpaid invoice, Diane1308.

When creating an credit memo, you can't enter a negative amount. That's why you're having that can't have a negative amount error. Please follow these steps on how to write-off a bad debt.

First, you'll have to create an expense account for the bad debt.

Then, make an item and then on the Account drop-down used the account you created earlier.

Next, create a credit memo and used the bad debt item.

Lastly, on the Available Credit window, select Apply to an invoice and then OK.

To check if you have successfully applied the credit memo, check on your customer's profile.

1. Go to the Customers menu, then choose Customer Center.
2. Look for your customer.
3. By this time, your customer's balance is zero.

I'm just a post away if you have other concerns with your QuickBooks file. Take care always!