Fatal error: Call to undefined function wp_installing() in /homepages/…./wp-ad

I got this message: Fatal error: Call to undefined function wp_installing() in /homepages/41/d119338305/htdocs/pangalactic/wp-admin/admin.php on line 92

This is line 92 … but what is wrong??

if ( ! wp_next_scheduled( ‘delete_expired_transients’ ) && ! wp_installing() ) {
wp_schedule_event( time(), ‘daily’, ‘delete_expired_transients’ );
}

After Login a HTTP ERROR 500 appears … after opening the website in a browser (Opera) it opens in a partial version of backend. I am logged in but no backend function will run. All buttons generate the HTTP ERROR 500. Only the logout works.

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