Provide us with WP Dashboard access to your site, please.
I see that builder is disabled at this moment on your site. Also, if you want to add custom tabs to your product you can use 2 options:
1) custom tab for all the products (Theme Options > WooCommerce > if you don’t use builder then Single product > tabs > add the custom tab title and custom tab content )
2) Edit product and add the Custom tab title and custom tab content to get the individual custom tab for the product.
If you want to disable More Offer tab follow the multivendor plugin docs https://wc-marketplace.com/faq/how-to-remove-more-offers-tab-in-the-single-product-page/
One standard license is valid only for 1 project. Running multiple projects on a single license is a copyright violation.