Deletion not possible; &1 date(s) already completed or fixed

What causes this issue?

Deleting a reminder rule is allowed only if none of the reminder dates
belonging to it has been designated as fixed or completed (the <ZK>Fixed
</> or <ZK>Done</> indicator is set).

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Some reminder rules allow you to enter an end date for the list of
dates. To keep the dates that have been fixed or completed up to now,
change the end date.
To delete the entire reminder rule, select all of the dates and remove
the <ZK>Fixed</> and <ZK>Done</> indicators.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.