It means that 'now' is not suitable/convenient. However, any other time is. A shorter way of expressing the same idea is 'not now'. If someone is trying to get your attention and you are very busy, you might say 'not now'. Meaning I am really busy right now, so wait until some other time.

Anytime but now = Anytime other than now = Anytime except now. Your friend: Can you help me? You: Sure! Your friend: When? Can you help me right now? You: Anytime but now. How about in an hour? Is that okay? Your friend: It sure is! Thanks!

I think it does mean now or never