you cannot apply this entry because it is closed proplem why

When i need to reverse entry from g/l entry then function-reverse

it give me message entry no xxx not applied

i go to customer card to this customer then select Customer - ledger entries select entry show that need to apply

then press apply it give me message error "you cannot apply this entry because it is closed proplem why "

why this message show and how to solve this proplem

this entry i need to reverse made in 26-05-2014

but i dont know what is the meaning message above

"you cannot apply this entry because it is closed proplem"

thanks