No entry without a main structure found for book structure &1

What causes this issue?

An assignment for &V1& already existed. You have changed the usage type
of <LS>Substructure</> into <LS>Book Structure</> without refreshing
the existing assignment. Therefore no entry without the main structure
can be found in the business function assignment.

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?

To refresh existing assignments, select the line and choose <LS>Entry ->
Refresh Main Structure(s)</>, or choose <LS>Entry -> Repair</>.
You can also change the usage type of the structure in the info object
list.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.