Hello,
Firstly, we have confirmed from the Test Site the Frequently Bought Together (FBT) is working fine. Here is the Product from the Test Site where you can check for yourself too: https://blackpearlspro.com/woodmart_support/product/augue-adipiscing-euismod/
On your Site, we are seeing multiple third party Plugins on your Site. Could you please once try deactivating all the third party Plugins on your Site remaining the Theme required ones activated. If the issue gets resolved, then reactivate them one by one to see which Plugin is causing the issue.
If somehow issue still persists, then please take complete Backup of your Site first, then provide us the permission to completely debug your Site which includes deactivating all the third party Plugins and removal of on-Site Customization changes too, so that we can thoroughly check this issue on your Site and assist you accordingly.
Best Regards
.
