\documentclass{article}
\usepackage{luacode}
\def\mycommand#1#2 {
\luadirect{
lettersmatrix = {L = {'L','b'}, K = {'c','d'}}
function find_letter (letter_to_find)
  for key, value in pairs(lettersmatrix) do
    for seckey, secvalue in pairs(lettersmatrix[key]) do
      if secvalue == letter_to_find then
	    return key, seckey
      end
    end
  end
end
key,_=find_letter(#1)%this doesn't find the letter?
tex.sprint('\luatexluaescapestring{#1-#2} ')%prints L-orem ipsum, correctly
%tex.sprint(key)%! LuaTeX error [\directlua]:1: no string to print
}}
\begin{document}
\mycommand Lorem ipsum dolor sit amet some random text some random text some random text some random text some random text some random text some random text
\end{document}