Is this sentence OK? Is the "now" at the end of the sentence redundant?
"It's too late now" is a common idiom meaning that the opportunity to do something has passed, presumably never to return. You could reasonably argue that "It's too late" (without the 'now') means that it is too late in the day to start something, but the task could be attempted if you start at an earlier hour on the next day, although context would definitely matter in the determination of meanings. (Alternatively, you could say "it's too late right now", which you would generally follow up with "come back tomorrow around 10:00" or some such rescheduling effort.)
So I would say that if your intention is to say that you can no longer perform the task at all, the sentence is fine as it stands.