If the Church cannot err in morals, how did it engage in the slave trade and write papal bulls approving it?

If and when they did that, it was rare, but that is not infallible stuff. The Catholic Church is like a lot of other institutions; we have plenty of screwballs who get promoted to this or that level, and this is Earth. The doctrine of infallibility applies to faith and morals when the Church teaches what is the right thing to do. You could theoretically have a pope making an infallible statement, say that slavery is wrong, and another pope disobeying it; that is possible, and it is what people are saying about Pope Francis today, since all the popes and councils laid down that Jesus Christ is the only way, truth, and life, and Francis seems to want to change that. He could be a material heretic; we are going to have to wait it out until he dies, because the next pope can say he was a material heretic, which is how the Catholic Church works. So popes make mistakes all the time, but when it comes to a dogma of the Church, called upon under the gift of infallibility, then we know it is infallible and cannot go wrong. People can disobey it afterward, even a pope, because he is not impeccable, but the infallible statement remains infallible.