You can only view a private YouTube video if the owner explicitly shares it with your Google account
Ask the video owner to add your Google email address to the video’s private sharing list
Make sure you are signed in to the same Google account that was granted access
Open the private video link while signed in
If you still cannot view it, confirm the owner shared the correct account and that you are using the correct login
If the video is unlisted instead of private, use the direct link shared by the owner