You must first set up a new account for bad debt before you can write off bad debt or write off invoices in QuickBooks Online. Go to the customer or vendor profile, choose the bill or invoice you want to write off, and then click "Create Credit Memo" from the menu. Apply the credit memo to the account you created for bad debts to finish. To get to know more about this "how to write off bad debt in QuickBooks online", read this blog. You can connect with QB professionals by dialing +1(855)-955-1942