6-31G(D) doesn’t (according to the Basis Set Exchange, anyways) define a basis set for Iodine. Switch to a basis set that supports Iodine. You can find a list on the basis set exchange. The question of “which basis set should I use?” depends, as always, on what your electronic structure method is. For DFT, high accuracy results require a thorough benchmark study to determine the most appropriate functional/basis combination at an acceptable speed.
Thank you for your comment, I found “Basis Set Exchange”. Do everyone download a missing bases file, e.g. 6-31G(D) for “I”? or, switch basis only for “I”, which is something like below? I’m not sure whether it is proper settings or not