Hisuian Zoroark

I was battling someone, and I used Hisuian Zoroark’s Doom Curse move. My opponent proceeded to retreat his active Pokémon and knock out the Zoroark . However, the Doom Curse did not seem to have any affect, and no Pokémon were knocked out. Was this a glitch or something that I do not understand about the Doom Curse? Also, does the knock out of Doom Curse count as an effect of an attack?