property assignment to a lease

Hi

I have property master defined at all the levels, building, floor and office.  In particular lease i would like to assign the entire floor area.  I am able to do this using

main lease->locations tab

But when i try to enter the billings after selecting the same location, the area/sqm field is non enterable.  Usually the amounts are entered using area/sqm field..

I would like to know any particular logic is there for this field to be active ?

We are using version 12.1.3

Please let me know if i am making anything wrong..

Thanks

Krishnan