get_current_site() is a function for getting the current Multisite Network. As with other multisite-related functions, it is not available to use if your site is not configured as a multisite network.
I can’t tell from your code what you’re attempting to use the function for, so I can’t offer a suggestion for what to use instead.
If you want to check if you’re on a specific page you can use is_page() along with the slug of ID if the page you’re checking for:
if ( is_page( 'about-us' ) ) {
}
if ( is_page( 5 ) ) {
}