Barley Miso substitute?
Miso is (usually) just a flavouring agent, so you almost certainly can substitute any other type of miso.  The flavour will of course be slightly different, but it is not going to ruin the recipe.

Barley miso that has been fermented for a long time is quite dark with a very strong flavour, so you may want to increase the amount of white miso or other miso to taste.