I’m the maintainer of Omnipay. What you get is an error because you’re not providing the transactionReference to the completePurchaseRequest. This is not an issue with Omnipay or Mollie, but with the integration.
You should receive a transactionReference in your fetchPurchaseResponse (see https://github.com/thephpleague/omnipay-mollie/blob/a8534c97464c4be59db15a3e48f8f3b3634b7a8b/src/Message/FetchTransactionResponse.php#L101-L1096)
You can store that in the database/session/whatever and use it in your completePurchaseCall, together with the other parameters.