Temporarily switch to a default WordPress theme, such as Twenty Seventeen for example, and then you will be able to activate your personal license for Visual Composer via the Visual Composer > Product License menu item in your admin sidebar. Then switch back to Vellum once you have activated the license.
Alas, we cannot activate that Product License tab in Vellum because, if we do, it prevents customers without personal licenses from updating the plugin. That’s the way WPBakery has chosen to implement it, there is nothing we can do about it.