WordNet sense 1 (not susceptible to):

insusceptible, unsusceptible

WordNet sense 2 (lacking the necessary skill or knowledge etc.):

unqualified

WordNet sense 3 (not qualified or suited for a purpose):

incompetent

WordNet sense 4 ((usually followed by `to') lacking necessary physical or mental ability):

unable

WordNet sense 5 ((sometimes followed by `to') not meeting the requirements especially of a task):

inadequate