You need to export your ThisVid login cookies. Use a browser extension like "Get cookies.txt LOCALLY" for Chrome/Firefox. Save the file as cookies.txt .
If a video is set to "Private" (visible only to the uploader), no downloader on Earth can get it unless you have the uploader’s password. The video file never reaches your browser. thisvid private video downloader new
You must be a "Friend" of the uploader with a logged-in browser session. You need to export your ThisVid login cookies