Hi there dmcinnes-selkirk,
Thanks for reaching out about your journal entries. Quickbooks Online is a great tool able to assist in keeping track of sales and expenses so you can see the profitability of your company. I can provide some information about journal entries.
One possible cause for this is if you added sales tax to one or multiple lines of the journal entry. This will add to either your credits or debits, which could make the credit and debit totals not be balanced. If this has been verified, and the error, I recommend closing the journal entry and recreating it.
if you have any other questions, or need further assistance, feel free to reach out here.