Obviously these things are not spoken of Melchisedec, who was not descended from Abraham, and who did not belong to any of the twelve tribes of Israel; they are spoken of Jesus Christ (Hebrews 6:20). But Christ did not belong to the one tribe in Israel from which priests were allowed to serve under the law, the tribe of Levi.
What great confidence we can have in our God to arrange all the parts of his plan in place! How carefully it has all been worked out! The Scripture cannot be broken, and we come with confidence to learn from it, certain that if we seem to see a problem, it is because of the weakness of our understand, not because there are contradictions in the divine plan.