Please restart your computer and try again. If it still fails, can you please follow these steps and try again?
1. Open Internet Options from the search window.
2. Under General Tab, click Delete in Browsing History and follow prompts.
3. Close Internet Options.
Once you are done resetting your Internet Options, please follow these steps:
1. Using Chrome, Firefox or Edge, go to YouTube.com and play any video and click the Downloader button. (This action will still show "Unable to download" message)
2. Close all Windows
3. Restart your PC
Now try your video. If the issue persists, please let us know.