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Prologue creation error?

I'm stuck with some kind of error that I really don't understand in Prolog. I get this error when calling a rule (does it work sometimes?):

error(instantiation_error,Var0)

Can someone explain to me what this means?

so I have two rules:

special(X) :- user(X, Days), Days >= 20.

special(X) :- premiumuser(X).

user(guy, 40).

although the user is added dynamically, sometimes it is not in the Prolog db. I can eliminate the first special (x) rule and the error.

when calling:

special(someone).

I am getting the error

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This means that you have applied a functor that only works on atoms (for example <

) with an unknown variable. You must post the minimum code that demonstrates the problem before we can tell you more.



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