Please do these steps to resolve your issue.
1. Open Internet Options from the search window.
2. Under General Tab, click Settings in Browsing History and click View Files. (This will open a folder with a lot of files. Select all and delete them).
3. Close Internet Options by clicking Ok.
4. 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)
5. Close all Windows
6. Restart your PC