The most common reason that this error message is shown is due to the expiry of the transaction link or another session of the transaction link is in progress.
For Merchants: Please make sure that the transaction is still valid (i.e. not expired yet) and make sure to create one new transaction per customer transacting
For Consumers: Please ask the merchant for a new transaction link if you are still not able to access the transaction after a few tries.
If the problem persists, please contact firstname.lastname@example.org