Souls do not enter in to Heaven until they have been purified so that they may be in union with God. So we sin lots on earth, then, provided we haven’t died in a state of mortal sin, we go to Purgatory, where we are purified. Once we’re purified, we’re done with sin and it’s onward to Heaven from there. So that’s why there wouldn’t be sin in Heaven—because you’re ready to be with God.