Cannot edit purcahse order quantity column in SAP b1 8.8

Hello experts,

I am facing a small problem...I have created a list of items in the item master. I enter those items in the purchase order blanket.. So by default the quantity is set to 1 for the corresponding item.. When i try to edit the quantity column a message is flashed saying " Cannot edit".. now when i enter the same quantity using the calculator in the quantity field i am able to get the desired number on the row..but again i get a message saying that "cannot edit" .

I am doing this entry in the manager mode.If i do the same process using another user id which do not have any managerial rights, i do not get any sort of error..

Kindly help me solve this problem

Regards

Shyam