how to ad twitter sharing button to hueman theme?

Please update Hueman to the latest version.
Or try Super Socializer Plugin

can it not be done by adding some code to single.php file?

As far as I know, this should be possible by default.
If it doesn’t display it by default, it will not display it when you insert the code manually. (assuming you’ve enabled sharing option)

You are currently using Version: 2.2.3, so please update to Version: 2.2.5 first to see if this solves the problem.

Super Socializer Plugin allows you to manually add share and like buttons as well.

thanks FadingMusic i will update it..