If you are incapable of something, you cannot do it — not “I failed this once,” but “this is not in me,” or “this thing simply cannot happen.” English almost always wants incapable of + a noun or -ing: incapable of swimming, incapable of telling the truth, incapable of running the new software. Though actually, it is a rather heavy, formal word. Friends more often say can’t or not able to; we reach for incapable when the inability feels complete, built-in, or slightly judging. About people it can sound cold: calling someone incapable at work often means you think they are useless, not just stuck on one task. About machines and systems it is calmer: a phone incapable of 5G. In official and legal writing it can mean a person cannot, in the eyes of the law, manage their own affairs. The opposite is capable. Do not pair it with to do.