If you have modified theme files/plugin -> You are not supposed to delete the files or directly modify them. if you have done that it’s recommended to switch the theme/plugin and delete the theme/plugin which was modified. Then again install it by searching.
That index file is actually placed to prevent someone from accessing the files directly
if ( ! current_user_can( 'install_plugins' ) ) { wp_die( __( 'Sorry, you are not allowed to install plugins on this site.' ) );
The code above means the only user who has the capability install_plugin will be able to install a plugin. As an admin, you will install the plugin as all admins have the that capability
I keep getting an “installation failed” error when trying to load a plugin.
There are many reasons for this to happen. Most of the WordPress directory plugins will be functional. If the plugin is not developed as per WordPress standards then it can fail on installation. It will show error as well. Check if you are able to install some popular plugins or try updating the WordPress by going to dashboard > update again.