Navbar menu cannot expand on mobile screen

Hi all,

I have a dashboard build with flexdashboard and hosted on github page. Everything work well except when on mobile screen, my navbar button is not expanded when it's clicked. I can not figure out what is wrong, since my layout code is pretty same with example dashboards available on flexdashboard documentation, and they all work well.

Anyone have idea what's wrong?

this is my dashboard with the source code embedded.
https://idrusfachr.github.io/covid19-flexdashboard/

Thank you!