Welcome back to the thread, RP2522. Thanks for keeping us updated.
I'm here to guide you on how to make the expense non-billable in QuickBooks Online (QBO).
You receive the error message "This transaction is linked to an invoice. Saving this transaction would make the invoice no longer valid. In order to make this change, edit the invoice first, remove this transaction from it and remake this change." since the invoice is linked to the billable expense.
To make the expense non-billable you'll have to remove the linked billable expense charge first to the invoice before unchecking the billable option on the expense transaction. To do this, here's how.
- Locate and open the voided invoice.
- In the upper left-hand, click the 1 linked Billexp Charge link.

- Click Remove.
- Add a new line item.
- Select Save and close.
Once done, go back to the expense transaction and make the expense non-billable.
For future reference, here's an article in case you'd want to run reports to view a snapshot of your business: Run reports in QuickBooks Online.
That's it! If there's anything else you need help with in managing your transactions in QBO, let me know by commenting below. I'll be more than happy to answer them for you. Have a good day!