Hi Guy,
I am facing the same issue. I have 8 lines in SO(print1). 3 lines are invoiced and those 3 lines are present in intercompany SO and PO(Those intercompany SO and PO are invoiced)
Remaining 5 lines are opened and those lines I have to cancel but when I tried to cancel it I am getting the below error
Quantity ordered may not be reduced because there are not enough inventory transactions with 'on order' status.
I try to cancel the quantity through delivery remainder but still getting the same issue.(print 3)
Picking list registration tab is greyed out.