Without knowing anything about the circumstances it's impossible to suggest why you seem to be having difficulty.
You need to ensure that you're within the time limits, that the item is covered by eBay's MBG and that you paid through eBay checkout by a method of payment that is eligible.
If you still have difficulty, ask again in the morning, but PLEASE give more information about exactly what you're trying to do and what is preventing you from doing it.
