I managed to install the theme as follows:-
Installing Theme from uploaded file: neptune-by-osetin.zip
Unpacking the package…
Installing the theme…
Theme installed successfully.
But Preview will show a blank page and Activation caused the website unusable until theme changed via mysql .
My wordpress version is Version 4.6.1.
Are you trying to install the theme on a WordPress fresh installation? Is not, please disable all plugins, one-by-one and see if the theme can be activated without errors.
Is the theme purchased from ThemeForest?
If you purchased the theme from ThemeForest you can ask the author for help if the theme can’t be used on a WordPress fresh install.
One last solution, go to your site’s root folder and open up wp-config.php and set the wp_debug from FALSE to TRUE. Then open the site where you get blank page and see if you get any php errors.
Please let me know the results
Problem fixed after changing the PHP to version 5.4.