Any console errors (client, or server, either way) when server starts, or client joins, or client takes damage?
Also, likely nothing to do with it not working, you'll probably want your health check to check less than 6, instead of == 5.
As is, it's quite possible someone could get hurt and hit 1, 2, 3, or 4 instead, which wouldn't help anyone with your current code needing to be exactly 5 health.
As a dev learning G-lua, those errors are extremely useful.
Learning to decode them can sometimes take a while though.