I have upgraded my clients file to 2016.2 this morning and when i went into the new file I could not apply a credit against an existing invoice either through Sales Register or through Receive payments. When you go into the Credit and go apply to sale it then brings up a new screen as you expect and then no invoices are shown which is the problem. There are open invoices shown in the register but you cannot apply any credit to them. When you do a new dummy invoice and a new dummy credit these will work as normal but a new credit won't apply to the old invoice nor can a new invoice apply to the old credit which might have been a workaround.
MYOB Support said delete everything and do it again as the answer. A bug like this should be checked before the upgrade.
Peter Davies