Please deactivate all the third-party plugins activate the parent theme then clear cache and check back.
If the problem continues then leave all the third-party plugins deactivated and provide with the admin panel login details of the website with the permissions to debug then we will troubleshoot with the theme plugins.
i did all you asked and i found out the theme is conflicting with a plugin that is fundamental to my business working.
i do a wholesale business and the “Advanced product quantity” from woo-commerce sets the minimum quantity i.e the minimum quantity a client can add to their cart for a certain product and also the plugin fixes the number of increment by a certain factor.
the client can add a min quantity of 6 of product A and they can only buy that quantity in steps of 6,12,18,24 etc
its hard for me to get rid of that plugin so can you help fixing that.