I am trying to post an invoice and it say's this bill number already exists. However, I can't find it anywhere...any suggestions??

Welcome and thank you for taking the time to reach out to us here on the Community page, @accounting413.

I'm glad to guide you today so you can locate the duplicate entry of your bill in QuickBooks Online (QBO).

Once the Preferences is set up to warn any duplicate number used for your transactions from the Accounts and Settings page, you'll get a message that it already exists from your records. You have the option to save it anyway or update the number to avoid duplicates. If you want to find the identical bills recorded, follow these steps:

1. Open your QBO account, then select Expenses at the left pane and select Expenses.
2. Go to the No. column to sort it. When the Expenses populate, it toggles the numbers in ascending and descending order.
3. Check for duplicates from there.

If you want to review any duplicate numbers used for your customers' invoices, follow the steps provided under the If you need to check for duplicate sales (invoice, sales receipt, etc.) numbers: section from this article.

In case you need additional references about managing your transactions while working with QuickBooks in the future, check the topics provided from this link.

If you have any other questions, mention my name in the comment section below. I'm always here to help and get back to you as quickly as I can. Keep safe always!