What kind of half-assed robot can't use contractions anyway?
They're in the dictionary, they're real words.
He either doesn't like to use them, or likes to lie about not being able to.
I can see how using contractions could allude a robot, it can allude many humans since there are situations where a contraction doesn't work as well as its counterpart. But the impression I got was just that Data simply "didn't" use contractions rather than "couldn't." But the show took it pretty far in that latter direction at times suggesting Data was simply incapable by way of programming to use contractions, which could have just been another "flaw" Soong built into into the "Data Generation" of Soong-type androids to make them "less human" in order for them to not as easily scare the colonists.