How to make reservation on requisition line

Within Navision Attain 3.60 we use reservations. I setup every item with reservation set to ‘Always’. After inserting a new salesorder-line, the system reserves from current inventory first. When all available items are sold or reserved, reservation is made from existing purchase orders. In our former Navision 2.50 version the next step was : If the required quantity is still not covered, new requisition worksheet lines are created on request to be reserved. Is this possible in Navision Attain 3.60, and if yes, how, because I can’t find the way to do it. If I try to reserve a requisition worksheet line Navision says ‘Reservation Illegal’ Thanx