Thank you! It seems I had Java-11 installed (althought it seems that not only the headless one, it was simply installed as a dependency). After looking through Synaptic I realized there are actually several versions. Having installed version 17 and 21 (only ones I could find) I managed to launch the program, Napoleonics seems to work, so I guess this is it!
Thank you, @uckelman and @cholmcc for replying, and of course thanks @uckelman for patience and your help in solving this problem. Linux can be difficult for a layman to operate, help from the community is always appreciated!