Incorrect spelling, explanation: this idiom, at least, is made from two separate words. Therefore writing it as one word is a mistake – there is no such word as atleast. It is a phrase, so it is written separately: at least.

Correct spelling, explanation: this form, at least, written separately, is correct because this phrase is made of two separate words. This is an idiomatic expression, so it is created by setting together some words (in this case two), but not combining them into one, so atleast is an incorrect form and at least is correct.

Definition of at least:
idiom, as much or more than; finally
I would like to earn at least $60k a year.
You could at least call that you’re coming!