minitest.rb

def solution(a)
  n = a.size
  prefix_sum = [0]
  
  a.each do |e|
    prefix_sum << prefix_sum.last + e 
    
  end
  
  for i in (0...n) #{}"range excluding n"
    left_sum = prefix_sum[i]
    right_sum = prefix_sum.last - prefix_sum[i + 1]
    if left_sum == right_sum
        return i 
      
      end    
  end
  
  -1
end

require 'minitest/autorun' 
class TestDemo < Minitest::Test
  def test_sample
    a = [ -1, 3, -4, 5, 1, -6, 2, 1 ]
    assert_equal 1, solution(a) 
  end
  
  def test_empty
    a = []
    assert_equal -1, solution(a) 
  end

  def test_no_answer
    a = [1, 1] 
    assert_equal -1, solution(a)
  end
  
  def test_left_corner 
    a = [0, 1, -1]
    assert_equal 0, solution(a)
    
  end
  
  def test_right_corner
    a = [0, 0]
    assert_equal 9, solution(a), "hello"
  end
  
  
  
end

That's awesome!
Line 24 should be:
class TestDemo < Minitest::Test

That was an unusual autocomplete... I also had a stray n before (0...n)

It works now :)