Reports

You're correct! It's because of the key "assignable" being false for the category ID 42 which is for YouTube shorts.

Using this documentation I used the following code and got the same information as you.

request = youtube_client.videoCategories().list(
    part="snippet",
    regionCode="US",
)
response = execute_request(request)
print(response)

So if I were you I'd choose another category unless you absolutely wanted to create a video with a category ID of 42.

Maybe refer to these answers for creating a YouTube short via the API.

79490501