I have a rails 7 app. Here are the relevant pieces of code:

/app/mailers/team_mailer.rb

def final_survey_nag_email
  @leader_name = params[:leader_name]
  @leader_email = params[:email]
  @team_name = params[:team]
  mail(to: @leader_email, subject: "Your Teams Progress")
end

spec/mailers/previews/team_preview.rb

def final_survey_nag_email
        @leader_name = Team.last.leader_name
        @team_name = "Team Candler"
        TeamMailer.with(team_name: @team_name, leader_name: @leader_name, subject: "Your Teams Progress").final_survey_nag_email
    end

/app/views/team_mailer/final_survey_nag_email.html.erb

 Hi <%= @leader_name %>! A couple of months ago you created the team <b><%= @team_name %></b> for the Team 

When viewing a preview of the email above at rails/mailers/team/final_survey_nag_email @leader_name is displaying a value, but @team_name is not. What am I missing?

you are setting team_name here

TeamMailer.with(team_name: @team_name, leader_name: @leader_name, subject: "Your Teams Progress")

but then you try and access team

 @team_name = params[:team]

so you should fix your preview to look for params[:team_name]