Even slightly better is to first check both

    if not API_KEY and not CHANNEL_ID:
        print("Missing both YOUTUBE_API_KEY and YOUTUBE_CHANNEL_ID.")
        exit(1)
    if not API_KEY:
        print("Missing YOUTUBE_API_KEY.")
        exit(1)
    if not CHANNEL_ID:
        print("Missing YOUTUBE_CHANNEL_ID.")
        exit(1)

That way you don't end up fixing one just come back and be told you're also missing another requirement