Simple prolog program. Getting error: >/2: Arguments are not sufficiently instantiated

Question

I made a Prolog predicate posAt(List1,P,List2) that tests whether the element at position P of List1 and List2 are equal:

posAt([X|Z], 1, [Y|W]) :-
   X = Y.
posAt([Z|X], K, [W|Y]) :-
   K > 1,
   Kr is K - 1,
   posAt(X, Kr, Y).

When testing:

?- posAt([1,2,3], X, [a,2,b]).

I expected an output of X = 2 but instead I got the following error:

ERROR: >/2: Arguments are not sufficiently instantiated

Why am I getting this error?

Answer 1

A Prolog predicate is a relation between arguments, and your statement

the element at position P of List1 and List2 are equal

is clearly an example where multiple solutions are possible.

?- posAt([1,2,3],X,[1,5,3,7]).
X = 1.

So the answer from sharky, while clearly explains why the technical error arises, requires a small correction:

posAt([X0|_], Pos, Pos, [X1|_]) :-
    X0 == X1.

Now it works as expected.

?- posAt([1,2,3],X,[1,5,3,7]).
X = 1 ;
X = 3 ;
false.

Writing simple predicates for list processing it's a very valuable apprenticeship practice, and the main way to effectively learn the language. If you are incline also to study the available library predicates, here is a version using nth1/3 from library(lists)

posAt(L0, P, L1) :-
   nth1(P, L0, E),
   nth1(P, L1, E).

This outputs:

?- posAt([1,2,3],X,[1,5,3,7]).
X = 1 ;
X = 3.

Could be interesting to attempt understanding why in this case SWI-Prolog 'top level' interpreter is able to infer the determinacy of the solution.