Declarative arithmetic in exercism

I’m pretty inexperienced with prolog and most of the resources I can find seem to assume having access to, for example, (#=)/2, which we don’t seem to have here. Is there a library or other workaround available?

Did you try putting

:- use_module(library(clpfd)).

at the top of your prolog file / the online editor?
If you use swi-prolog you should have access to it. There are other prolog interpreters out there and while I am sure they may have access to the library or have it built in.

CLPFD is a library that is sadly not auto-imported nor part of the prolog ISO yet. So make sure you include the library yourself