I think the problem is the Casper-Two hasn’t been updated to work with Hugo 0.58 (and newer).
The problem is
which no longer works. It should use .Site.RegularPages
instead of .Data.Pages
.
I think you should open an issue for the theme developer on GitHub, or fork the theme, update it and send in a PR for consideration. The second option is probably best since it seems the theme’s developer is no longer active. Or see if any of the forks are in current development.