1. tz-issue: you must wrap the timezone in quotation marks: .tz('America/Chicago')
2. "The type of result changed from number to text." - please read this (the paragraph above the video): https://help.podio.com/hc/en-us/articles/201019358-Calculations
3. The calculation:
var frq = @all of frequency.toString();
var frq = frq == "Daily" ? "days" : frq == "Weekly" ? "weeks" : "months"
var lastDate = @max of date of contact;
moment(lastDate).add(1,frq).tz('America/Chicago').format('MM DD YYYY')
Replace @all of frequency with the correct name of the category field.