Is there a reason that I can't undo buying an elixir?

is it after you are 6 items and then buy it ? cuz then no cuz you consume it as soon as u by it

That's like asking for a refund for a can of soda that you just finished drinking.

A couple items are like that as far as I know. It is because as soon as that item is bought, it can have an impact on the game. For example, you could buy blue elixir, hit champions with long range spells, then undo it and buy an item. Its a very niche situation, but Riot just catagorized them in with items like that to prevent abuse cases and to be consistent.