Material &1: Max. 9999 sched. lines possible in sched. agmt. &2, item &3

What causes this issue?

For technical reasons, the maximum number of schedule lines is limited
to 9999.

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?

There are several ways to solve this problem. You should work through
these in the given sequence:
Re-number the schedule lines in scheduling agreement &V2&, item &V3&.
<DS:TRAN.ME85>Execute Function</>
Aggregate the schedule lines in scheduling agreement item &V3&.
<DS:TRAN.ME86>Execute Function</>
Create a new scheduling agreement item. Make appropriate entries in
source of supply determination, so that the system considers the new
item instead of the old item for automatic scheduling agreement
schedule lines.

Error message extract from SAP system. Copyright SAP SE.