Hello,
01. To set the brand attribute on a variable product with existing attributes:
a. Go to Products > Attributes and create a “Brand” attribute.
b. In the product’s Attributes tab, add the “Brand” attribute alongside your existing attributes.
c. Select the brand for the product.
02. 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 let me know I will further check on your site and give you a possible solution.
Best Regards