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What causes this issue?

You want to plan the field group "Current value" manually and the field
group "Maximum value" automatically (or vice versa) using these
planning parameters.
In order to ensure that a current value greater than the maximum value
can never be entered, 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.

Error message extract from SAP system. Copyright SAP SE.