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What causes this issue?

You want to plan the field group "Planned activity" manually and the
field group "Capacity" automatically (or vice versa) using these
planning parameters.
In order to ensure that the planned activity value can never be greater
than the capacity, you must always plan both field groups manually in a
planning activity.

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?

Please flag both field groups, or
unmark both.

Error message extract from SAP system. Copyright SAP SE.