The Magisterium of the Church teaches that anyone who is not Catholic cannot be Pope

 

“Cum ex Apostolatus Officio” – Apostolic Constitution of Pope Paul IV About the Danger of Heretical Authorities

Catholic Encyclopedia 1914 (Vol. 11, p. 456):

“It is quite clear that the election [as Pope] of a heretic, schismatic or woman would be null and void.”