Hi Jonah,
Apologies for the delay in my response. Have you tried to enter your details manually because there will be times that you will have to enter your details in to your account? I have written a little how-to guide below if you are stuck.
1. Log in to your "Manage My Booking" account
2. Click on the "Your holidays" tab along the top
3. On this page you will need to add your - Booking Reference, Departure Date and Postcode
4. Click on the "Find bookings" button
5. The website will then be able to find your booking and your holiday will appear
From this page you should be able to select a departure, in this case your Croatia trip, and pay any outstanding balances.
Give this a try and if you have any more questions or queries, please let me know.
Kindest regards,
Monty