Cannot spawn wither skeletons by using spawn egg or /summon command when the mod installed. Even the config related to skeletons all disabled.

need mods and version specification

Game version is 1.20.1,but i'm not sure which mod causes this

Ok, from personal testing, this is a mod conflict, I tested this myself, wither skeletons spawn fine. I then tested it with Mob Variants mod (which was previously incompatible) and they still spawn fine. Mind checking if there are any mods that modify wither skeletons that you have enabled? That would help fix this.