Welcome to the Community.
Let me share information about reconciling an account in QuickBooks Online.
When reconciling, make sure to review the dates of the transactions. It should be within the reconciliation period.
If not, you can edit the date of the reconciliation period. See the screenshot below for the reconciliation period.

For future reference, you can read this article about reconciling an account: Reconcile an account in QuickBooks Online.
If you encounter an error when reconciling, here are articles to help you:
Let me know if you have questions about reconciling an account. I'm always here to help. Have a great day.