The method `add_template_helper` does not seem to work in Rails 6.1
So, apparently, this method was deleted without deprecation warning in this commit: https://github.com/rails/rails/commit/cb3b37b37975ceb1d38bec9f02305ff5c14ba8e9 So, the fix for this issue is to replace: class MyMailer < ApplicationMailer add_template_helper MyHelper With: class MyMailer < ApplicationMailer helper MyHelper