How do I fix this health game-pass script?

The code will creat this error every time due to the code:-

local humanoid = game.Workspace:FindFirstChild(player.Name)
if humanoid == nil then
	print("Failure to set health to " .. humanoid) -- error
else
-- other code

As you check that the variable humanoid is nil you then concatenate the string Failure to set health to with humanoid which is nil causing this error each time.

print accepts a varying number of arguments allowing you to list the thing you want to print.

print("Failure to set health to ", humanoid) -- no error
print(typeof(tostring(nil))) -- nil to string representation

The code above will not error as tostring will be called on each argument passed to the print function.

With this in mind the error can be resolved but your code itself can be simplified by using the correct events such as CharacterAdded.

-- use local variables and functions where possible
local marketPlaceServ = game:GetService("MarketplaceService")
local gamePassId = 5207158

game.Players.PlayerAdded:Connect(function(plr)
	
	-- check for game pass
	if marketPlaceServ:UserOwnsGamePassAsync(plr.UserId, gamePassId) then
		print(plr.Name, " Has game pass ", gamePassId)
		
		-- connect a player added event to give the game pass hp
		plr.CharacterAdded:Connect(function(charModel)
			local humanoid = charModel:WaitForChild("Humanoid")
			humanoid.MaxHealth = 150
			humanoid.Health = humanoid.MaxHealth -- an easy way to give max hp
		end)
	end
end)

I hope this helps.