OkiOki automatically picks up Paypal receipts from your mailbox and links them to the corresponding transactions. But a Paypal receipt in itself is not sufficient as evidence to be able to recover your VAT, for example. Ultimately, you still need the invoice for that.
That is why, OkiOki will never automatically set an entry with only a receipt and a transaction to ready. She also expects the corresponding invoice. If she finds this in your mailbox or at your suppliers, OkiOki will try to link it to the receipt. And after that she marks the booking as done.
If the invoice is still missing, the receipt will appear on your to-do list as a reminder of the missing invoice.
If you don't have the invoice, you can always manually mark the booking as done (provided the transaction is there).