As a project manager, we are the team's leader. It's easy to feel that we should have all the answers.
Leadership is not knowing everything.
Leadership is knowing where to find the knowledge and the answers. It's OK to say "I don't know." It's OK to ask for help from your team. After all, the project team is there to work together with the project manager.
As a leader, admitting that you don't know will make you look more human in the eyes of your team. It will also create an opening for someone on the team to show their value by answering the question.
Saying "I don't know" creates opportunity for creativity. If I don't know how to build an interface screen, it means I have to learn how to do it, to find inspiration from good examples of interface design. This is a great opportunity to work with my team and build something new and fresh.
Not knowing can be leveraged to your advantage.