Is it permissible to wear a string necklace with the letter "S" (the first letter of my name) on it if common custom does not consider this imitation, and does this entail being cursed by God?

If it is established that wearing necklaces has become a prevalent custom for both men and women in your country, then it is not considered imitation. However, if this attire is predominantly worn by those who commit sins, then wearing them is prohibited. It must be noted that those who commit sins and are promised a curse are exposed to it, even if they have other good deeds, and they are subject to the will of Allah.