Redirect non existing page to frontpage

You can use conditional tag is_404() and wp action hook to redirect whenever the page can not be found.

functions.php

add_action( 'wp', 'se344018_redirect_404' );
function se344018_redirect_404()
{
    if ( is_404() ) {
        wp_redirect( home_url() );
        //
        // wp_redirect( home_url('some/page-slug') );
        exit;
    }
}