“Là” is an adverb refering to a place, “Y” is a pronoun that is often used as a place complement.

You could say “Ne l’y mettez pas”, or “Nous irons là ce soir”, but the meaning is slightly different.

While “Là” refers to a place, “Y” refers to the place where something is.

For instance:

“You see this restaurant? I go there!”

1. “Tu vois ce restaurant? J’y vais!” -> It suggests that you go inside the restaurant.
2. “Tu vois ce restaurant? Je vais là!” -> Here, you go to the place where the restaurant is, not necessarily inside.

Can you see the difference now?