I believe you wanted to set up a confirmation_instructions.html.erb file from app\view\resource\mailer , when usually the resource is a user or administrator
First of all, make sure you set config.scoped_views = true to config/initializers/devise.rb . Thanks to this, the installer looks for templates in your resource view folder, and not by default in app\views\devise\mailer
Using login , username , etc. possibly as follows <%= @resource.login %>
In the end, be sure to restart the server. For reference, check out a similar Ruby / Rails issue : how do you customize Devise mailer templates?
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