Simulation planned orders cannot be changed manually

What causes this issue?

The planned order that was selected is a simulative planned order from
long-term planning.

System Response

Planned orders that are created in long-term planning cannot be changed
or firmed. They are used to check the effects of the simulative demand
program on production and procurement. Thus, a preview is availble of
the planned demand program.
The planned orders are changed or replanned in the operative planning
run.

How to fix this error?

You can only change the simulative planned orders from the results
screen of the long-term, single-item planning.

Error message extract from SAP system. Copyright SAP SE.