It seems to make sense to use UP TILL NOW only in a negative sentence (to describe a situation that has existed up to this point or up to the present time). => I wasn't available up till now.

From my understanding, they're both correct, but there are slight differences that might not matter that much. "I have been busy up till now" indicates that you were busy for a certain (perhaps relatively long) period of time that started sometime in the past, but you're free now in the present. "I was busy until now" indicates that you were busy sometime in the past, but you're free now in the present. I think the contextual difference that I feel is purely dependant on the speaker's (primarily emotional) perception of the duration of time in which the activity took place, but I'm not certain if others share that same feeling. Also, in my opinion, "until", "up until", "till", and "up till" all fit in each sentence and the meaning remains the same; however, the "up" might be redundant as it doesn't really add anything.

Hello Asia, The first one is best, the idea of using the present perfect for actions in the past related to the present. The second one doesn't sound right at all. You would use this if you were talking about a known time in the past. 'I was busy yesterday' Best wishes Bob

If you are saying until now, I would tend to go with the present perfect (I have been busy) as it links to now. However, as you are using the present perfect, it kinda makes until now redundant. So, I might just say have been busy. Or maybe, I have been busy recently.