“I'm sorry, I can't go into that.”
You're leading a meeting at work. One of the attendees at the meeting asks you a question. He asks about some information that you're not supposed to share, so you refuse to answer the question.
I'm sorry, I can't go into that.
"Going into" a topic means to speak about it.
You can use this phrasal verb to talk about what someone said:
Did she go into any details about our new health benefits?
You can use this phrase to talk about what you're saying as it's happening:
Do you want me to go into a few more details?
There's another phrase which is similar:
I don't want to get into it.
"Getting into" a topic means talking about something that's complicated or annoying to explain.
This is a very clear, straightforward way to say "no" to someone's request:
A: Can't I have the afternoon off?
B: I'm sorry, there's no one else to take your shift.
Another way to refuse that's a little softer is "Sorry, but unfortunately..."
A: Can I have the afternoon off?
B: Sorry, but unfortunately there's no one else to take your shift.