Triskaidekaphobia

Triskaidekaphobia was, as stated by Nardole, a fear of the number 13. The Twelfth Doctor assured his companion Bill Potts that the fear was irrational. (COMIC: A Confusion of Angels)

Behind the scenes
The reference towards the "fear of 13," although relevant to the story itself, is additionally also a joke about the controversies surrounding Jodie Whittaker being cast as the Thirteenth Doctor. The dialogue was featured in the final issue of Titan's ongoing Twelfth Doctor series, likely explaining the inclusion.