Thanks for dropping by in the Community space and posing your concerns, pamzahoran5.
I'll outline some handy details to assist you in adding tags to bills in QuickBooks Online (QBO).
To locate those bills that have already been paid, you can follow the steps below. Here's how:
Go to the Expenses from the left menu.
Choose the Tabs section.
From there, select the Paid tab.
Find the bill that you're looking for and click the dropdown icon.
Tick View/Edit bill.
From there, you can now add tags while modifying the paid transaction. Once done, click Save and close.
You can also see this reference for further information about using tags and group tags. This way, you can get deeper insights into your sales and expenses: Tag transactions in QuickBooks Online.