How can i implement a reward-chest using .loc?

I tried

var = <object path>
var.loc = locate(usr)

But it's like: "Cannot modify /obj/item/booster/exp_boost_bottle.loc"

What should i do, except quitting dm, pals?

Sounds like you're trying to modify a type's loc. Create the object, then set it's location.
var/chest/booster/item = new/obj/item/booster/exp_boost_bottle

item.loc = locate(usr)

item.loc = locate(usr)

The above amounts to:

item.loc = locate(usr.type) in world

I don't recommend this. At the least, you could set loc directly to the container (mob or whoever). However, if you wanted to use locate(), a proper way would be using a tag to locate said container.
I think you need to post more of the code in question, so the context is clear. Your original post is obviously pseudocode, and that doesn't really show us what you're doing.