Cannot cancel a sales order line -- Line locked at time of processing

 Hello,

When I try cancel a sales order line on an order which is in booked status Iam getting the below error.....Can you please suggest what could be the problem and how I can resolve the issue

Line locked at time of processing

The sales order is in booked status and has 3 lines...The 2 lines are in closed status and the 3rd line which I want to cancel gives me the error message below when I try to cancel it..

Line locked at time of processing

Thanks,

Kiran