The people who say "I don't know" are some of the brightest and smartest I know. It shows you're willing to learn, humble, and not pompous and elite.
"I don't know - I need some time to research the effect of XYZ on our prooposed solution. Let's get together tomorrow."
"I don't know. Let me ask Bill from the platform team if he has any suggestions and then get back to you."
etc.
Weak performers use "I don't know (how to/where to/ etc)" as an excuse to give up; strong performers use it as an opportunity to learn without bucking responsibility.