Humanoid won't take damage?

I'd recommend using either region3 or raycasting, but Magnitude works alright.

With what you have done, you are connecting the touched function to the part with every loop. This isn't a good thing.

Secondly, you were using a for pairs loop for a singular part. This won't work because it isn't a table.

local part = game.Workspace:FindFirstChild("Part")
local maxdist = 7 --max distance from the part


local function search()
	
	local children = workspace:GetDescendants()
	
	local characters = {}
	
	local humanoids = {}
	
----------------------------------------------------------------	
	for _, v in pairs(children) do
		
		if v:IsA("Model") then --grabs models only
			
			if v:FindFirstChildOfClass("Humanoid") then
				if v:FindFirstChildOfClass("Humanoid").Health >0 then
					table.insert(characters, #characters + 1, v)
				end
				
			end
		end
	end
----------------------------------------------------------------		
	
	for _, v in pairs(characters) do
		--checking the distance between parts
		local primarypart = v.PrimaryPart

		if primarypart then
			local distance = (primarypart.Position - part.Position).Magnitude
			if distance <= tonumber(maxdist) then
				table.insert(humanoids, #humanoids + 1, v)
			end
		end	
	end
----------------------------------------------------------------	

	
	for _, humanoid in pairs(humanoids) do
		--damaging the humanoids
		local human = humanoid:FindFirstChildOfClass("Humanoid")
		if human then
			if human.Health >0 then
				human:TakeDamage(50)
			end
		end
			
	end
----------------------------------------------------------------	
	children = nil
	characters = nil
	humanoids = nil

end



while part ~= nil do
	wait(0.5)
	search()
end

Maybe test this script out. If it isn't to your liking, then I'd suggest looking up both region3, and raycasting.

https://developer.roblox.com/en-us/articles/Making-a-ray-casting-laser-gun-in-Roblox https://developer.roblox.com/en-us/api-reference/datatype/Region3