Disable checkout page for not logged in users

Solution:

Is possible to redirect non logged customers that trying to access to checkout with this code:

add_action( 'template_redirect', 'checkout_redirect_non_logged_to_login_access');
function checkout_redirect_non_logged_to_login_access() {

    // Here the conditions (woocommerce checkout page and unlogged user)
    if( is_checkout() && !is_user_logged_in()){

        // Redirecting to your custom login area
        wp_redirect( get_permalink( get_option('woocommerce_myaccount_page_id') ) );

        // always use exit after wp_redirect() function.
        exit;
    }
}

Then you can display a custom notice in cart page with a button linked to login area, to avoid customer frustration. Is better to warn customer before, than after.

// Displaying a message on cart page for non logged users (Optional)
add_action( 'woocommerce_before_cart', 'customer_redirected_displaying_message');
function customer_redirected_displaying_message() {
    if( !is_user_logged_in() ){
        // HERE Type your displayed message and text button
        $message = __('To access checkout, you need first to be logged in', 'woocommerce');
        $button_text = __('Login area', 'woocommerce');

        $cart_link = get_permalink( get_option('woocommerce_myaccount_page_id') );

        wc_add_notice(  $message . '<a href="' . $cart_link . '" class="button wc-forward">' . $button_text . '</a>', 'notice' );
    }
}

Code goes in function.php file of your active child theme (or theme) or also in any plugin file.

The code is tested and works.