This isn’t a matter of grammar, though. It’s simply that these are two things that mean the opposite of each other. A negated sentence, that is, a sentence with the word the word “not” added to it, tends to mean the opposite of the original sentemce. Compare these two example sentences:

I like pancakes.

I do not like pancakes.

Or these two examples:

I could swim across.

I couldn’t swim across.

In the same way, the sentence “I could care less” means the opposite of the expression “I couldn’t care less”. But due to how similar they sound, “I could care less” has, for a long time, been a common mistaken way to invoke the usual expression of “I couldn’t care less”. But they are not interchangeable, as they literally mean the opposite of each other due to the difference of “could” vs. “couldn’t” (contraction of “could not”). When you say “I could care less”, you are actually saying the exact opposite of what you most likely intend.

Perhaps it helps to break down each sentence in your head, as if you had just heard them for the first time, and try to make yourself conscious of what each of them actually means and expresses at their most basic. I’m sure you will find that only “I couldn’t care less” actually expresses a sentiment of “not caring” about something, whereas “I could care less” says that you actually do care about something, but is unclear about whether you care a lot or only a little.

Sorry if I’m overexplaining here, I’m just trying to find the words that best make it make sense to you. :)