FunctionClauseError is what I’d expect from iteraterefs if it was called with number < divisor. You likely need to define it for those inputs as well to pass Exercism’s tests.

General note: this isn’t going to do what you want. i = i + 1 rebinds the name i inside the do / end block but that value doesn’t escape or even make it to the next iteration.

Same thing for out = out <> elem(table, i) inside iteraterefs; the name out is bound to a new name but then the scope ends immediately.

My recommendation would be to forget completely about Enum.each for a little while; it is almost never the right solution in Elixir.

I went away, far up into the mountains which are the Elixir documents, I learned many things and when I returned to take on the challenge again, it neatly folded before me like the recursion I used to solve it:

defmodule RomanNumerals do
  @doc """
  Convert the number to a roman number.
  """
  @table {"M", "CM", "D", "CD", "C", "XC", "L", "XL", "X", "IX", "V", "IV", "I"}
  @refs {1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1}
  @spec numeral(pos_integer) :: String.t()
  def numeral(number) do
    iterate(number, @refs, @table, 0, "")
  end

  defp iterate(num, refs, table, index, out) do
    if num == 0 do 
      out
    else
      ref = elem(refs, index)
      cond do
        num >= ref -> sym = elem(table, index)
                      iterate(num - ref, refs, table, index, out <> sym)
        true -> iterate(num, refs, table, index + 1, out)
      end
    end
  end 
end

Nicely done, a big improvement! Probably the only other change I’d advocate for is to do this:

  defp iterate(0, _refs, _table, _index, out) do
    out
  end

  defp iterate(num, refs, table, index, out) do
    ref = elem(refs, index)
    cond do
      num >= ref ->
        sym = elem(table, index)
        iterate(num - ref, refs, table, index, out <> sym)
      true ->
        iterate(num, refs, table, index + 1, out)
    end
  end

It’s conventional that when you’re doing recursion like this to define the “base case” or “termination case” as its own clause up front, and then you have other clauses after. This is entirely a stylistic thing though, so it’s up to you!