There were two mistakes:

line 79: min_af_td_sched was called out as a string instead of a variable

variable line 98: min_af_td_sched should be converted to a string with .to_s before writing to the idf file.

Despite these mistakes being fixed, I was still unable to successfully run the measure using OpenStudio version 2.9.1. The measure does work with the new OS version 3.0.1 (App version 1.0.1).