When I run this, there is a something wrong in the sum+=n statement, I don't know how to fix it - ruby

Question

class Test
def multiples
(1..1000).each do |n|
if n % 3 == 0 || n % 5 == 0
sum += n
end
puts "The sum of multiples of 3 or 5 below 1000 is #{sum}"
end
end
end
test = Test.new
test.multiples
Error:
test.rb:9:in `block in multiples': undefined method `+' for nil:NilClass (NoMethodError)
The sum of multiples of 3 or 5 below 1000 is
from test.rb:7:in `each'
from test.rb:7:in `multiples'
from test.rb:19:in `<top (required)>'
from -e:1:in `load'
from -e:1:in `<main>'

Answer 1

You have not initialised sum, adding sum = 0 should fix this:
class Test
def multiples
sum = 0
(1..1000).each do |n|
if n % 3 == 0 || n % 5 == 0
sum += n
end
puts "The sum of multiples of 3 or 5 below 1000 is #{sum}"
end
end
end
test = Test.new
test.multiples

Answer 2

I remember when I started out in Ruby, I didn't really observe the error messages much, but you might find value in learning how to read those messages. You have:
test.rb:9:in `block in multiples': undefined method `+' for nil:NilClass (NoMethodError)
Just to dissect the obvious a little here. You get the following parts:
test.rb - Your file name
9 - The line number where things break down.
block in multiples - There's a block with an issue
inside 'multiples'
undefined method '+' - Remember that a + sign
is actually just a method name in Ruby and different methods are
available for different variable types. This is saying there's no
method by the name of + for the variable type you're calling it on.
nil - This is the variable type you just tried to call the +
method on.
By element 5 of that error message, you should be thinking "Why is sum currently nil?" And then look around your code and realize you've never declared it. Ruby doesn't know how to add something to nil, so you should immediately think about instantiating/declaring the variable 'sum' somewhere outside of the loop that it's adding things to itself.
Hopefully understanding error messages and why you had an error helps. Because if you don't understand and think through an error message, you're going to hit a road block again. It will probably be around the same time you try to create an array by shoveling something into it, instead of instantiating it first, and wonder why you get a similar error message.
Hope this helps!