TikTok only gives this stream to logged-in viewers, so the server (which has no TikTok login) is refused. That's why it plays fine in your own browser. Paste a TikTok sessionid cookie in Settings below and press Check again.
Where to find it: open tiktok.com logged in → F12 → Application → Cookies → https://www.tiktok.com → copy the sessionid value. Only this cookie is stored, on your own server — never your password. A secondary account is fine (it must be 18+ verified). Logging out of TikTok in that browser invalidates it.
Paste a link, then press Download
Save to:browser default
The video is fetched by the server (no watermark) and sent straight to this device — nothing is kept on the server.
Drag to find the moment, then download and upload it in YouTube Studio as a custom thumbnail.