General Lack of Charm:
* Uninspiring
* Dull
* Boring
* Uninteresting
* Bland
* Monotonous
Lack of Leadership Qualities:
* Incompetent
* Ineffective
* Uninspiring
* Weak
* Unassertive
* Unconvincing
Lack of Personal Appeal:
* Repulsive
* Unattractive
* Off-putting
* Unpleasant
* Unlikeable
Lack of Authenticity:
* Phony
* Fake
* Insincere
* Artificial
* Superficial
Lack of Influence:
* Powerless
* Weak
* Ineffective
* Uninfluential
The best antonym will depend on the specific situation and the way in which charisma is being used.