Incorrect spelling, explanation: suffix -able is generally used with words that, without a suffix, are in their full form (just like understandable without the suffix is understand – we don’t change anything). Convert is such a word, but it is an exception. Since its Latin-origin word we use the suffix -ible with it, so convertible is the correct form.

Correct spelling, explanation: usually with Latin-origin words we use the suffix -ible when creating an adjective. Convert is one of such words, so convertible is the correct spelling. It is sometimes said that suffix -able is added to words that are left full without the suffix (such as depend → dependable), and this is generally true, so that may be why the confusion with convertable exists, however, this rule has exceptions, and convertible is one of them.

Definition of convertible:
adjective, something adaptable, able to be changed
Look how convenient this convertible bag is – it can be a handbag or a backpack!
noun, a type of car with a roof that folds
Joe was planning to buy a coupé, but finally, he bought a convertible.