You do not actually need a license key if you only download 10-20 videos per month.
Ensure you don't have extra spaces at the beginning or end of the string when pasting. license key for video downloadhelper new
: A single license can generally be used on all your personal devices. However, the license may temporarily lock after approximately 8 new installations (e.g., after PC refreshes) to prevent unauthorized sharing; it can be unlocked via a link sent to your registered email. Google Groups Recovery of Lost Keys If you have misplaced your license key, you can use the official license recovery page You do not actually need a license key
If you download videos (once a week or less), the free version might suffice, or you should look for a free web-based downloader to avoid the watermark. the free version might suffice