Correct spelling, explanation: at least is an idiom, so we should write the words separately. Atleast is incorrect, because in idiomatic expressions we do not mash words together. Therefore at least is the correct form.

Definition of at least:
idiom, more than or no less than; finally
At least I found the perfect dress!
Please buy at least 2 kilos of flour when you’re at the shop.

Incorrect spelling, explanation: atleast is an incorrect form, because as a phrase, the words in it should be written separately. Therefore the correct way to spell this expression is at least – with a space between words. At least is the only correct form, because this idiom is not one word.