This is the difference between an adjective (a word that describes something) and a noun (a name of a thing).
Preventive is an adjective, as in “Perform preventive maintenance routinely and your car will treat you right.”
Preventative is a noun, as in “An aspirin a day is a good preventative to guard against heart attacks.”
The word “preventative“ is often used incorrectly in place of “preventive.”