"Reservation cannot be made" error message when trying to book a train from Berlin to Gdynia

If that’s the case, is it possible to buy the tickets ahead of time? I’m worried if I’m only going to buy the ticket in-person at the DB ticket office once I’m already in Germany which would be too last minute and risky. I thought there would be a risk of tickets being sold out or tickets being too expensive because last minute price fluctuation. How do people usually deal with this matter if they prefer a peace of mind by planning their trip months ahead, booking reservations early?

Do you need a ticket or a seat reservation? If you have an Interrail/Eurail pass you only need a seat reservation and those always cost the same. 

@seewulf Do you have more information on the reservation between Germany and Poland?

I believe I need both. I don’t plan on getting the Interrail/Eurail pass, and I plan on buying individual journey tickets instead. From what I understand, i need to have tickets for both connection (RE1 from Berlin to Frankfurt(order) and EC59 from Frankfurt(order) to Gdynia), and I also need a seat reservation for EC59 and not on the RE1. Please correct me if I’m wrong. Ideally, i just want to buy a ticket and reservation for the whole journey online and ahead of time for convenience, but I’m noticing it’s not that straightforward. Can you advise me on what’s the best way for me to deal with this matter? Thank you!

If you buy normal tickets the compulsory reservations are included. 

I see. Is the only way to buy normal tickets through in-person DB ticket office then? because the error message that popped up when I’m trying to buy through the DB website. 

What’s the travel date? Tickets become on sale 60 days in advance on this route (59 for the best Sparpreis fares). I tried a random June date on DB Navigator and it worked fine.

It’s going to be on July 23rd. This exact journey:

I presume it has to do with the works that week (train starts in Frankfurt (Oder) instead of Berlin). The following days show the same message.

The Polish website doesn't offer anything yet for that date either. I'd check back in a few days.