The Amplified version of the bible gives you a clearer picture of the passage--a fuller look into the Greek words and their meaning.


24 [Even] now I rejoice in the midst of my sufferings on your behalf. And in my own person I am making up whatever is still lacking and remains to be completed [[v]on our part] of Christ’s afflictions, for the sake of His body, which is the church.


It isn't that Jesus' sacrifice on our behalf was lacking, it's that WE must do our part and endure suffering for the advancement and benefit of the Body, the church.