It looks like you’re trying to check for a Part’s Property named: Health, can you double-check your script to make sure you’re truly trying to find the humanoid?
You’re trying to access .Health from a HumanoidRootPart. You should call :FindFirstChildWhichIsA("Humanoid") on the zombie to get its humanoid instead.
PS: you can use zombieHuman.Health -= DMG to subtract health instead.