Need help with wither-like potion effect

I'm fairly new, and I'm trying to make a potion effect that's like wither, and that deals a custom damage type. I made some basic code and slapped it on active tick condition in the hope that it worked. No matter what I've tried it doesn't deal damage. I even set to generic damage to see if that could work. I haven't found any guides on this so either everyone knows how to do this and it's obvious, or I'm doing too deep.

any help is great.

image of my code here: https://imgur.com/a/gnPiImf