The start can only be moved within the first period

What causes this issue?

You want to move the start of the separation allowance event to a
different month.

System Response

You cannot move the start of the event to a different month as the
periods for the separation allowance have already been created and
sequentially numbered. You can only move the start date within the
first period.

How to fix this error?

If the start of the event is in a different month, cancel or delete the
old event and create a new event for the correct dates.

Error message extract from SAP system. Copyright SAP SE.