"Now I rejoice in what was suffered for you, and I fill up in my flesh what is still lacking in regard to Christ's afflictions, for the sake of his body, which is the church."
Something is indeed lacking in regard to Christ's afflictions. It is our share in them.
Paul was simply saying that the suffering he went through didn’t even come close to the suffering Christ went through. To claim that as the insufficiency of Christ’s suffering and sacrifice or that we must add to Christ sacrifice is nonsense at best.