I added tf2 keys but wasnt seeing by usable credit.Writing on key credit 4.2452 , but if I join for buying game , writing usable key credit 0.8639 , can you help me ?

Please check your market order tab and see if there is a "Battleblock Theater" buy order. You can cancel it and unlock the credit.

I think an empty buy order was created when you were loaded into a cached page.