Key date scheduling not possible

What causes this issue?

You have attempted to perform key date
<GL:maintenance_plan_scheduling>scheduling</>.
Key date scheduling is not allowed for performance-based strategies.
In the case of time strategies, all <GL:maintenance_package>maintenance
packages</> or cycles must last at least a month or a multiple thereof.

System Response

The System automatically removes the indicator <LS>Key date scheduling
</> and sets it to time scheduling.

How to fix this error?

Check the <GL:preventive_maintenance_strategy>maintenance strategies</>
or the cycle. Select another scheduling indicator, for example, time/
/factory calendar.
If you are already in the scheduling function, you must restart the
maintenance plan. The plan is scheduled as a time-based maintenance
plan.

Error message extract from SAP system. Copyright SAP SE.