Note that purchase requisition &1 &2 must be manually adjusted

What causes this issue?

You are using dynamic assembly with planned orders and have converted a
planned order into a purchase requisition. You now want to change the
sales order item (for example, order quantity or date).

System Response

The purchase requisition cannot be adjusted automatically.

How to fix this error?

Adjust the purchase requisition manually.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.