Field group &1 cannot be deleted

What causes this issue?

You want to delete field group &v1&. This field group contains all
fields that are not assigned to a special field group. Therefore it is
particularly necessary.
You cannot delete the field group.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.