Invalid reference calendar

What causes this issue?

The reference calendar for the selected procedure package is blank or
is not maintained for shifts or does not cover the complete planning
horizon.

System Response

The system terminates the planning run.

How to fix this error?

In Customizing, under
,,,,APO -> Master Data -> Calendar ->
<DS:TRAN./SAPAPO/CALENDAR>Maintain Planning Calendar (Time Stream)</>,
maintain a planning calendar with a calculation rule that is defined
for shifts. Make sure that the calendar extends long enough into the
future.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.