The API (advance passenger information) could have been given at any time after making the booking. It's at least possible that the fact you are only inputting this information now might be part of the reason for the error.

Have you tried inputting the API information in the normal "manage my booking" environment? I don't know if it's even possible at this late stage (now that OLCI is available, the booking is under airport control) but it's something you can try. If you've given all the necessary information related to your booking and your ID, it may be that a subsequent attempt to check in online is successful (i.e. a "normal" check in where you don't need to fill in this extra information as part of the check-in flow).

In any event, you can just check in at the airport tomorrow. It sounds like they don't have your full passenger details, so they will want to see your passport etc.