Her exact words are that Macbeth is "not without ambition, but without the illness that would attend it." Some people have argued that this line suggests that Macbeth is ambitious, but the line "art not without ambition" isn't quite that. If I say my friend is coming to play football next week, and someone asks if they're any good and I say: "Well, they're not
not good" you should probably manage your expectations regarding how good they actually are. In many ways, this is really Lady Macbeth saying that, in fact, he's not really that ambitious at all.