QuickBooks Online doesn’t have an ‘Undo Last Reconciliation’ feature.

To undo a reconciliation, you’ll need to unreconcile each transaction via the Register.
1.Select Transactions on the left menu then Registers.
2.Click on the account then click Go to Register
3.Find the transactions that need to be unreconciled, one at a time.
4.To highlight […]

Resolution for 'Bank Reconciliation Undo issue: 'Undo a Bank Reconciliation in QBO' available: Yes.

The Bank Reconciliation Undo issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the Bank Reconciliation Undo issue yourself by looking for a resolution described in the article. If it is a complex Bank Reconciliation Undo issue or you are unable to solve the issue, you may contact us by clicking here or by using other Bank Reconciliation Undo support options.

For more information about resolving issues related to Bank Reconciliation Undo in Quickbooks/QBO or to get support for the issue 'Undo a Bank Reconciliation in QBO', click the link below: