Hi Rakesh,
This is because of user status profile
Check whether any status profile assigned to your Credit memo in VOV8
and in the credit memo, check at line item-->Status Tab
if any system status is displaying, further Below in the tab, you can see OBJECT TYPE pushbutton
just click on that and set if you have any status, try to change this and check
this is the reason which is blocking the Billing
Further you can check in the configuration for your User status profile which is blocking ( transaction code OIBS)
try and revert if you need any further help
thanks
santosh