Thanks!

Thanks for your post but:
1)

lhs: 77, rhs: 'M'

Print your output... The kata is not responsible for the way Codewars print list of small numbers (it is the usual Elixir way! Unfortunately:-) You can avoid that if you output yourself your result.

Makevalley.make_valley

I can't change the class name since it will invalidate all solutions that already passed. Moreover I see nothing offputting in that:-)

3. I don't understand your intermittent failure.