Not all material assignments could be copied

What causes this issue?

Reasons for this could be:
The PM task list contains BOM items that were deleted in the relevant
BOM before you copied....
Changes to the BOMs and task lists were carried out with a change number
which can also lead to discrepancies between BOM items in the BOM and
in the task list.

System Response

The system reads the basic BOM in any case. If an item no longer exists
there, it does not copy the appropriate item from the task list.

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?

Compare the material lists of the task list to be copied with the copied
task list. If necessary, maintain certain material items manually in the
copied task list.

Error message extract from SAP system. Copyright SAP SE.