Depreciation key field is in display mode

Hi Gurus

i am selecting depreciation key for Tax bal in oayz

Area Depreciation area Deact Depkey Use Per Index Layout

01 Book deprec Y111 15 0 1000

15 Tax bal sht 15 0 1000

20 Cost -acc Y111 15 0 1000

Now i am unable to select dep key for tax bal sht. dep key field is in grey color.

please tell me solution it is very urgent

thanking you

Krishna