I can no longer upload videos to Niconico via CyberLink PowerDirector after June 2023. What's the cause of that and what can I do?

Because the 3rd-party online video service: Niconico is planning to terminate the connection access for all other 3rd-party PC apps (e.g., CyberLink PowerDirector) in July 2023, the Niconico video upload feature will be no longer available in all versions of CyberLink PowerDirector. The option has been removed in the new version of PowerDirector 2024/365 (released in Sept 2023) accordingly.

Here is the announcement of the service change from Niconico for your reference (in Japanese only):
https://blog.nicovideo.jp/niconews/178304.html

If you would like to upload your videos to the Niconico website, please consider producing your video project as a video file to your hard drive first in PowerDirector, and then upload the videos to Niconico in your web browser.