Hello,
Please follow this guide and make sure it is configured correctly for product.
https://xtemos.com/docs-topic/frequently-bought-together/
If the issue remains, Please deactivate all the 3rd party plugins and activate only theme-required plugins on the site and then check the issue. I am sure your issue will be solved. Then activate the 3rd party plugins one by one and check which plugin is creating the issue for you.
Otherwise, if the issue still exists, then keep the 3rd party plugins deactivated and share the wp logins details so I can check and give you a possible solution. Also switch the child theme to the parent theme.
Best Regards,