You need to import the constraint logic module.

  :- use_module(library(clpfd)).

Goes at the top of the file (leave off the :- if you're in the repl).

The arithmetic built in to prolog is traditionally of the form

  A is 1+2,
  B is A+3,
  ... etc.

GNU Prolog is an excellent choice for everybody. I once considered switching back and forth to SWI if I ever needed some convenient module GNU lacks. The need never really arose!