Requirements grouping indicator not allowed

What causes this issue?

During the check of the master data for the project definition and the
WBS element, one of the following inconsistencies was found:
Although the <DS:DE.GRONTOP>Automatic grouping</> indicator was set in
the project defnition,
Several WBS elements in the project structure were declared to be
<DS:DE.GRPSPKZ>grouping WBS elements</>.
The value of the field <DS:DE.GRPSPKZ>grouping WBS element</>was set to
2 for a WBS element.

How to fix this error?

Reset the automatic grouping indicator in the project definition or
change which WBS elements are grouping elements.

Error message extract from SAP system. Copyright SAP SE.