It really depends on the country/train, IC is a code used in different countries for long-distance non-high-speed trains with slightly different rules. Generally speaking, those trains have either no seat reservations or optional seat reservations (but no mandatory seat reservations).

Therefore I would assume you do have a ticket that's valid on your chosen train. For example, for the train between Belgium and Luxembourg you mentioned in the comment, a seat reservation is definitely not mandatory, most people won't have one and will take chances. I am not even sure they sell them anymore. And in the Netherlands (if you happen to go there) seat reservations are not available at all on domestic NS trains, even cross-country trains are really glorified suburban trains and there is no way to guarantee you have a seat (a first-class ticket can help however).

Where available, it is certainly possible to book a seat reservation separately but that's usually not possible online (the German railways do sell them online).

It really depends on the country/train, IC is a code used in different countries for long-distance non-high-speed trains with slightly different rules. Generally speaking, those trains have either no seat reservations or optional seat reservations (but no mandatory seat reservations).

Therefore I would assume you do have a ticket that's valid on your chosen trains. In some cases, seat reservations are available but not mandatory and open tickets do not mention the train for that reason. For the one specific route you mentioned, seat reservations are simply not available on IC trains at all. If you're really concerned about finding a seat, you could book a first class ticket. It doesn't guarantee anything either but there is usually more space in first class.

Where available, it is certainly possible to book a seat reservation separately but that's usually not possible online (the German railways do sell them online).