Thank you. I found the problem.
Now i want some help .... Let me explain my setup first- I have installed flarum in public_html/hacknation and moved the public files to public_html
This is my main public_html files after moving all files from public -

This is pubic_html/hacksnation directory -

And this is my pubic_html/index.php file -

And this is my pubic_html/hacksnation/site.php file -

My error is that site.php ( from pubic_html/hacksnation/site.php) creates a new directory for assets in pubic_html/hacksnation (2nd pic)... it is not using public_html for assets because the paths are not correct. So please guide me about the path directories.
(you can take a look to above pics for more clarity)
What should be the path for pubic_html/index.php and pubic_html/hacksnation/site.php for this setup to solve the error.