Saying ‘I can't’,’I don't know’,’I don't have time’, ‘I don’t understand’ is not a sign of weakness.
Not knowing isn't a problem, but hiding it is unprofessional and will cause considerable damage as it comes to light further down the line when it’s much more expensive to address.
Lead by example and create a culture by saying "I don't know", up front, out loud - then look around and ask "Who's got the answer?"
Show that you don’t lose authority or respect by admitting you don’t know or don’t understand - this isn't part of your domain, it's ok if you don't know.
You can't expect to know everything, but you can find ways to get the answers you don't know. That's a much more powerful and useful approach.
In fact, asking questions to understand what you don’t actually gives you more power because once you have the answer you know as much as the other person. Take them for all the knowledge they have.