@digitalcraftsman may know better, but I suspect that that theme is prematurely upgraded to the latest Hugo 0.16-DEV, which is long over due … It should be fixed in the theme so it will work with Hugo 0.15.
This is where the error points to: {{ .Summary | markdownify }}.
What is in your summary? If you have not designated the summary via the <!--more--> tag, Hugo takes the first 70 words of your post and designates them as summary.
However, I (the theme maintainer) fixed this issue in this commit.
If the issue is still present please leave a note in the linked issue on GitHub above. For other issues please contact me directly by creating another issue.