You might not be able to check in online for several reasons. For example, online check-in might not be open yet, as ferry companies have a specific timeframe within which passengers can check in online. If you’re traveling with a Greek ferry company, bear in mind that usually online check-in is available from 48 hours to 2 hours before departure and you cannot check-in outside this timeframe. Therefore, you should always check the travel information provided in the booking confirmation email.
Moreover, there might be some kind of system error preventing you from checking in or the ferry company’s website might be undergoing scheduled maintenance.
Please note that if you’re having any trouble with the online check-in process, you should get in touch with the ferry company that operates your trip, as they are the ones providing the online check-in service and they will be able to give you more accurate information on why the check-in failed and how to get your e-tickets.
Furthermore, if you are very close to the departure time, you are highly recommended to visit one of the local agencies, which are found in the "local travel agencies" section.