Availability control can not be activated for hierarchical projects

What causes this issue?

You execute a budget distribution of investment program position on a
WBS element.
There is at least one WBS element under this for which the following
applies:
The corresponding project is hierarchically structured, this means it
only contains level 1 WBS-elements.
The availability control is not yet active for the corresponding
project.
An automatic activation of the availability control is planned for
budget allocation in the corresponding budget profile (activation type
1).

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

For technical reasons however, the availability control <ZH>cannot</>
be activated automatically for the projects of WBS elements when the
budget is allocated.

How to fix this error?

You must activate the availability control for the projects of such WBS
elements afterwards if necessary (transaction CJBV).
Otherwise the availability control of such projects is at the latest
activated when the budgeting of these projects occurs <ZH>outside</>
budget allocation; in other words when the budget received within this
project through the budget allocation is distributed further.

Error message extract from SAP system. Copyright SAP SE.