Most likley one of these two issues apply:
1) You bought a non-Turbine product such as a Codemasters box. The keys inside a Codemasters box only work with the Codemasters Lotro Store. Unfortunately, Codemasters no longer operates a Lotro service. So these keys are worthless.
2) You bought a box with that has previously opened and the key used. Keys are single use. You have a low cost used box (even if it is not labeled that way by the seller). You wasted your money.
In both of these cases, you need to go after the seller to get a refund.
If you do not know why the key does not work. Call Turbine Account Management. They can tell what kind of key it is. The can tell if the key has ever been used. They should have a record of which account the key was applied to and when. They can manually apply the key to your account if the system is incorrectly rejecting the key.
Unless stated otherwise, all content in this post is My Personal Opinion.