Can't

Can't
 

p.111 And she explains to me that when I burst into the room I interrupted her at a moment when she can’t be interrupted.

When I was a student more than seventy years ago, our teacher told us could can refer to both the present and past; but can can only refer to the present, never the past; then why can't, not couldn't, above? Is it a typo?

Both "can't" and "couldn't" are correct.
"Couldn't" is used when the time of the event is being discussed.
"Can't" is used when the situation is being described. The "moment can't be interrupted."

Um...Do you mean because the situation is always 'can't be interrupted', not referring to a particular moment in the past, we can use 'can't'?

Yes, you understood it correctly.

Excellent answer. Thank you.