Hi there, nippers-clovelly.
Let's make sure to fix the error in your reconciliation in QuickBooks Online (QBO).
Can you share a screenshot of where you're seeing the error message? We just want to make sure that we will be giving you the right steps or information to sort this out.
Lastly, you may need these articles for additional reference in fixing reconciliation in the Online product:
Please reply to this thread to share more details. We look forward to it.