How to close window at byobu?

You can either type exit, or use Byobu's keybinding for killing windows/panes, which is Ctrl-F6.

Full disclosure: I am the author and maintainer of Byobu.

ctrl + a, then k

If it asks for confirmation, type y and hit enter.

If there are no more jobs running in the window, then you can use Ctrl+D to close it.