Hi Vineet,
I checked in my system. This is the normal behavior of SAP that disables (makes invisible) the Tax Amount field as system restricts the User to manually enter the tax amount when you are checking the 'Calculate Tax' indicator. This field will be enabled only if the 'Calculate tax' check box is unchecked.
This is controlled at the program 'SAPMF05A' and at the screen level '1200'. Please check the below modules which making the screen difference in FB70 when the 'Calculate tax' check box is ticked.
Image may be NSFW.
Clik here to view.
Image may be NSFW.
Clik here to view.
BR, D'nesh
