Colossians 1:24 Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking 14 in the afflictions of Christ on behalf of his body, which is the church,
When Paul was preaching to the Gentiles he had all mannor of physical suffering (may had to do with his prison time). What he is talking about here is not a deficiency of the Cross but his own personal physical sufferings that he suffered as he preached the “Good News”. He preached that Christ suffered on the Cross to atone for sin. Paul is preaching he accepts his physical ailments while preaching on behalf of Christ.