A member of the Flemish Group of Twenty, Henri van de Velde had an enormous influence on design and architecture. His only poster design was for .

- Tropon, a food supplement.

Explanation:

As per the question, the only poster design created by 'Henri van de Velde, the Belgian painter who had been a member of the Flemish Group of Twenty, was for 'tropon, a food supplement'. His art had a considerable impact on design as well as architecture. Therefore, his only poster made for the food supplement named 'Tropon' is also reflective of his artistic fervor. The beauty of his design made the product to become quite popular among the audience. Thus, 'tropon; the food supplement' is the product for which Velde designed his first and only poster.