الجواب وباللہ التوفیق

It is permissible to wear such a necklace which has the Ayatul-Kursi inscribed on it, there is no harm in doing that. However, one should make sure that the verses of Quran written on it, do not come in contact with the body when one is in a state of impurity (i.e. janabah) whether big or small. Also, one should make an effort to remove it from the neck while entering the toilet.

With the above mentioned conditions fulfilled, there is no harm in wearing such a necklace.