Average people fail not because they lack ability, but because they lack concentration.
Literal
Normal person [topic-は] ability [subject-が] insufficient [because-から] not [and-ではなく], concentration [subject-が] insufficient [because of-ために] fails.
~からではなく~ために is a contrast of two reasons: 'not because X, but because Y.' 集中力 = 'concentration/focus.' An aphoristic observation about the nature of failure.