QB Issue Resolution:

The check # should appear under REF NO.  You might have the REF NO. column hidden on the reconcile screen.  If you don’t see the REF NO column, click the small gear icon and make sure the Ref No box is checked.

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Resolution for Issue 'Reconcile bank account screen no longer shows check number & I don’t have a way to add that column. How can I reconcile a CHECKING account without showing CHECK number?' available: Yes (Solved).
Source: Intuit Community forum.

Reconcile bank account screen no longer shows check number & I don’t have a way to add that column. How can I reconcile a CHECKING account without showing CHECK number?: this 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 issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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