Sched. agreement item &1 &2 does not have a forecast delivery schedule

What causes this issue?

You want to create a <DS:GLOS.JIT_delivery_schedule>JIT delivery
schedule</> although a forecast delivery schedule does not exist yet
for scheduling agreement item &V1& &V2&.

How to fix this error?

Create a delivery schedule first in order to define the requirement
quantity for the medium and long term. Normally, the JIT delivery
schedule controls the delivery all by itself.
If the error occurred in the IDoc processing, you must not start
processing again until the IDoc that creates the forecast delivery
schedule for the scheduling agreement has been processed.
Afterwards, you can start the IDoc again.

Error message extract from SAP system. Copyright SAP SE.