Resources not being assigned to Bookings

According to https://woocommerce.com/de/products/woocommerce-bookings/, the plugin you are using is a commercial plugin for which you will find support here: https://woocommerce.com/contact-us/

I would find that strange. You have to open a ticket under the link above so that the support of this commercial theme can help you. Unfortunately, this is not possible here in the forum as no volunteer here has access to commercial plugins.