Everytime I click the 'Login' button it always directing me into 'auth.php' page, but I noticed that there's no file called auth.php? Is that missing file? Why did there's no auth.php?

Did you perform an upgrade from an LDU version? In Seditio, it is now handled by users.php?m=auth. If you performed an upgrade, there may be some links in your skin/tpl files that you must change to point to users.php?m=auth

Check that, and let us know how it works out for you. Hope this helps.

Thanks so much. I changed it to users.php?m=auth and it works

I'm glad I was able to help. If you have any questions or require any further assistance, please don't hesitate to contact us. Thanks for using Seditio.