Replacing a plugin function with a custom renamed function doesn’t work

First, in the original wc_bookings_get_time_slots_html() function, the following line:

$block_html .= '<div class="please-select">Hi please select your session</div>';

doesn’t exist, so that is confusing for people trying to get what is the difference between the original function, and the one you copied (renamed and customized) in your active theme’s function php file.

Now copying a plugin function to your theme (rename it and make customizations in it) will not do anything.

Instead use the available hook wc_bookings_get_time_slots_html, that will allow you to make your customization simply this way:

add_filter( 'wc_bookings_get_time_slots_html', 'filter_bookings_get_time_slots_html_callback', 10, 6 );
function filter_bookings_get_time_slots_html_callback( $block_html, $blocks ) {
    global $product;
    if ( 'customer' !== $product->get_duration_type() ) {
        $block_html="<div class="please-select">Hi please select your session</div>" . $block_html;
    }

    return $block_html;
}

Code goes in function.php file of your active child theme (or active theme). tested and works.