Menubar such as file,edit, view, project, settings are not displaying in kdenlive 21.04. I have tried ctrl+m which is not working. How to fix this issue ? I'm using ubuntu 16.04.

I assume Ubuntu 16.04 could be the problem. Try with the AppImage. If the AppImage is not working properly delete kdenlive-appimagerc (see here https://community.kde.org/Kdenlive/Configuration). If it's still not working you have to update to Ubuntu 20.04.