Hello,
Please edit the product >> Enable the Stock Management from the Inventory Tab >> https://ibb.co/ByXXZjJ and check the issue.
Make sure you have assigned the attributes in products, Edit the product for which you want to show the brand attribute and assign that attribute to the product – https://prnt.sc/QGfKsahh3O9a
If the issue remains, 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.
Best Regards