Why is this not possible? It doesn't make any sense. I saved up my credits only plan to buy some stock video for a project and now find that it was pointless. What possible reason could there be for not being able to use your credits for video?
Branching to a new discussion,
I see you have an Adobe account under the same ID as with forums.
I did go through your account details and noticed that you have Adobe Stock 10 images a month(one-year) plan. The subscription plans only entitle you to license standard assets.
Premium assets, such as videos, Premium collections images, and editorial images or videos, can be licensed on-demand or using credits from a credit pack.
You may also refer to Stock FAQ: What are Credit packs? to get more information on credit packs.
Feel free to update this thread in case of any additioanl questions.