I agree that it is difficult on its face. Catholicism requires that you believe in the Pope's authority and the Magisterium of the Church. And the social doctrine of the Church can be described as "Christian socialist." Libertarianism has as its base the belief that all humans themselves are sovereign and today at least if anti-socialist (I say today, because many protolibertarians, such as Lysander Spooner, also influenced socialism).
In addition, libertarianism grew out of classical liberalism, which was strongly anti-clerical. The Church returned the favor, rejecting "liberalism".
But Catholicism, does at its base believe that human beings are free. Paragraph 1731 of the Catechism says that:
Freedom is the power, rooted in reason and will, to act or not to act, to do this or that, and so to perform deliberate actions on one's own responsibility. By free will one shapes one's own life. Human freedom is a force for growth and maturity in truth and goodness; it attains its perfection when directed toward God, our beatitude.
Ultimately, the decision to do good therefore must be with the individual. If virtue is enforced, can someone truly be virtuous? These are issues I hope to further explore.