Superior functional location & does not exist (Edit --> F9 again)

What causes this issue?

You have entered functional locations for the lower levels of the
structure, but not an uppermost functional location.

System Response

The system warns you against this method of proceeding. When creating
<GL:functional_location>functional locations</> and
<GL:reference_functional_location>reference functional locations</>,
you should always start with the uppermost level of the structure.

How to fix this error?

If you want to deviate from the recommended procedure for any given
reason, you will have to select the function again.

Error message extract from SAP system. Copyright SAP SE.