You cannot set this scenario to operative; not all projects in pl. scope

What causes this issue?

If you change the operative scenario, the system normally does the
following:
Sets the marketing projects in the old operative scenario to inactive
Sets the marketing projects in the new operative scenario to active
In this case, some of the marketing projects are not in the planning
scope of your scenario set, so the system cannot set them to active or
inactive. For this reason you currently cannot set the scenario to
operative.

System Response

The system has not set the scenario to operative.

How to fix this error?

Add the missing marketing projects to the planning scope of your
scenario set. Add the marketing projects to the appropriate scenarios.
If the marketing projects do not belong in the scenarios, they are
probably in wrong place in the marketing hierarchy, so you need to
reassign them to a different higher-level marketing project.

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.