This issue could be happening because you may be using the older version of plugIn. Kindly update it to resolve the issue.
If your WooCommerce is already up to date, the issue can be due to the old WooCommerce template file included in your theme.
You can try replacing the file manually as described below or ask support from your theme vendor.
Please replace file
wp-content\themes\<your-theme>\<woocommerce template folder>\single-product\add-to-cart\variable.php
with the one