testing flash[:notice] with capybara and activerecord NameError: undefined local variable or method `flash'

I am very new to building websites and am currently running into the following problem.

I am using ruby, rspec, sinatra, sinatra-flash, capybara and ActiveRecord to design a website.

I am not using rails currently

I have implemented a user signup/login page whereby if the user does not fill up all the fields on sign-in a flash[:notice] should pop up. When I feature test this myself by using sinatra's rackup, it works perfectly. However, when I run rspec using capybara it raises the error:

NameError:
       undefined local variable or method `flash'

I cannot figure out why this would be. All my other tests pass.

My feature test code:

scenario "not filling a field, raises message 'Please ensure you have filled all the fields'" do
    visit('/')
    click_button("Sign up")
    fill_in 'username', with: 'ChrisL'
    fill_in 'last_name', with: 'Lovell'
    fill_in 'first_name', with: 'Chris'
    fill_in 'email', with: 'chris@chris.com'

    expect(current_path).to eq '/sign-up'
    expect(page).to_not have_content "Welcome, ChrisL"
    expect(flash[:notice]).to eq "Please ensure you have filled all the fields"
  end

My controller code(removed other routes):

class Chitter < Sinatra::Base
    
enable :sessions
register Sinatra::Flash

get '/sign-up' do
    erb :sign_up
end

post '/signing-up' do
  @new_user = User.create(
    username: params[:username], 
    email: params[:email],
    first_name: params[:first_name],
    last_name: params[:last_name],
    password: params[:password]
  )
  @new_user.save
        
  if User.find_by_id(@new_user.id)
    session[:user_id] = @new_user.id
    redirect '/'
  else 
    flash[:notice] = 'Please ensure you have filled all the fields'
    redirect '/sign-up'
  end
end
end

My erb file:

<!DOCTYPE html>
<html>
  <head>
    <title> Chitter Sign In/ Sign Up </title>
  </head>
  <body>
    <header>
      <h1>Chitter: <small>Sign In</small></h1>
    </header>
    <h2 style="color:blue">
      <%= flash[:notice] %>
    </h2>
    <form action="/logging-in" method="post">
      <label for="username">
        Enter Username
        <input type="text" name="log_in_username">
      </label>
      <label for="password">
        Enter A Password 
        <input type="password" name="log_in_password">
      </label>
      <input type="submit" value="Log in">
    </form>
      <br>
    <header>
      <h1>Chitter: <small>Sign Up</small></h1>
    </header>
    <h3> Sign Up For The Unique Experience </h3>
    <form action="/signing-up" method="post">
      <label for="username">
        Enter Desired Username
        <input type="text" name="username">
      </label>
      <br>
      <label for="first_name">
        Enter Your First Name
        <input type="text" name="first_name">
      </label>
      <br>
      <label for="last_name">
        Enter Your Last Name 
        <input type="text" name="last_name">
      </label>
      <br>
      <label for="email">
        Enter Your Email 
        <input type="text" name="email">
      </label>
      <br>
        <label for="password">
        Enter A Password 
        <input type="password" name="password">
      </label>
      <br>
      <input type="submit" value="Sign up">
    </form>

  </body>
</html>