Field group &1 (type SCGR) is ref. group of &2. &2 is not the root group

What causes this issue?

Field group &1 (type SCGR) is a reference group of &2. &2 is not the
root group (group with the highest hierarchy level).

System Response

Processing of the field groups was terminated.

How to fix this error?

Only use interface groups (group type = SCGR) as reference groups in
root group &3 (group with the highest hierarchy level). Adjust the
field group table correspondingly.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.