Saying "I don't know" in an interview isn't the killer most candidates think it is
Candidates who say "I don't know" and follow it with how they'd find the answer get more credit than candidates who bullshit their way through something they clearly don't understand.
IMO the second group is way more common. Long meandering answers full of buzzwords where everyone in the room can tell you're stalling. We're not impressed and we're definitely not fooled.
"I haven't worked with that specific tool, but I'd approach it by looking at the documentation and running a small test case first" is a real answer. It shows problem-solving instinct even when you don't have the knowledge yet.
Just don't overuse it. One or two honest "I don't know" moments build trust. Five in a row means you're underqualified.