Hi,
Welcome to the forum!
Siptah requires multiple assets from the base game, such as building pieces, thralls, pets, and recipes. Hence why it is not a “stand alone” game, and is rightfully described as an expansion.
Fortunately Funcom made this option available, and you can read more about it here;
If you are dissatisfied with the product you purchased you can learn how to request a refund on;
However, as you have been enjoying Conan with your family for “some time” your purchase is most likely no longer eligible for a refund.
Good luck!