Format grp fr. header information only valid for highest hierarchy level

What causes this issue?

You have entered or changed the <DS:DE.PCLASS_HDR>format group</> in the
header information.
Usually the <DS:DE.PCLASS>format group</> from the row information of
the superordinate group is used in reports. The format group from the
header information only applies to the highest hierarchy level, that is,
only for the group entered in the report directly.

How to fix this error?

Only enter the format group in the header information if you want to use
the group in the report directly. For groups at lower hierarchy levels
enter the format group from the row information of the respective
superordinate group.

Error message extract from SAP system. Copyright SAP SE.