Skip up to 3:00 mark I think.
You need to do a few things here.
Store the GameObject you spawnedYou're spawning a GameObject at runtime so you have to store it in a variable so you can reference it later.
Get your Health script from itUse Get Component to target that stored GameObject variable and get the component from it (Health script)
Change the Health variable in the scriptUse Set Property to target the variable inside the component you just stored from the Get Component action and set it to the value you want.
This is the process to do since it sounds like you're trying to use a Scene object to affect a spawned Prefab from the Project Hierarchy. However you can target components and child gameobjects locally inside a Prefab and it will work when you spawn it. The limitation is between Scene and Project, but it works fine if you drag n drop stuff and contain the linkage inside the Prefab, it can spawn and operate normally because all of the references are local.