Is there a way to make it so only premium players can have shift lock?

Try using the following in a LocalScript (make sure to put it in a place that LocalScripts can run, for example in StarterPack):

local player = game:GetService("Players").LocalPlayer

--If they don't have Premium, disable shift lock
if player.MembershipType ~= Enum.MembershipType.Premium then
	player.DevEnableMouseLock = false
end

I would try to help give an explanation as to why the code you provided didn't work, but it is very difficult to read due to the lack of indentation and codeblock. If you could correct that it would help immensely in solving your problem.

Hope this helps.

Edit: Just realized, if you're going to change the player's MaxHealth as well, use the following code instead:

local player = game:GetService("Players").LocalPlayer

--If they have Premium, give them more health. If they don't, disable shift lock.
if player.MembershipType == Enum.MembershipType.Premium then
    player.Character.Humanoid.MaxHealth = 200
else
	player.DevEnableMouseLock = false
end