input = " "
until input == 'quit'
    puts "
Menu
(1) Hi!
(2) Credits
(3 or quit) Exit"

    input = gets.chomp.to_s

    case input
        when "1" then
            puts "Hi!"
        when "2" then
            puts "Written By: wally"
        when "3" then
            input = 'quit'
        else
            puts "Invalid input"
    end
end

# => output

Menu
(1) Hi!
(2) Credits
(3 or quit) Exit
Written By: wally

Menu
(1) Hi!
(2) Credits
(3 or quit) Exit

# why this error?

..\Playground\:9:in `<main>': undefined method `chomp' for nil:NilClass
(NoMethodError)


Thanks

Did you read the definition of 'gets', for instance here?

    http://ruby-doc.org/core-2.4.0/IO.html#method-i-gets

It says:

    Returns nil if called at end of file.

Hence, your 'gets' encounters an EOF. You should verify the value
returned from 'gets', before processing it further.

> # why this error?
> ..\Playground\:9:in `<main>': undefined method `chomp' for nil:NilClass

I recommend to write code in such a way that you can handle nil objects.

Here is how I would rewrite it:

  loop {
    user_input = $stdin.gets.chomp
    case user_input
    when '1'
      puts 'Hi!'
    when '2'
      puts 'Written By: wally'
    when '3'
      break
    else
      puts 'Invalid input'
    end
  }