Mode &1 must contain ONE calendar resource

What causes this issue?

Only one calendar resource is allowed per mode. The working times of
this resource are used in scheduling. If the other resources listed in
the mode have different working times, they will not be taken into
consideration.

System Response

Orders cannot be created using a PPM if there is no or more than one
resource indicated as calendar resource.

How to fix this error?

Set the indicator for the calendar resource for only one resource in
the mode.

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.