Bill of material is not valid

What causes this issue?

You want to create or change a BOM item on a key date. However, the
task list header to which you want to subordinate the item, is invalid,
or not created, for this key date.

System Response

The item can not be created.

How to fix this error?

Select a key date on which the task list header is exists or is valid.

Error message extract from SAP system. Copyright SAP SE.