This means Google's bot detection is blocking you, usually after several failed sign-in attempts or because you're on a VPN. Try these in order:
Sign in to Gmail or YouTube in the same browser. This helps Google recognize you as a real person.
Turn off your VPN, then try again.
Clear your browser's cache and cookies, then try again.
Not sure how to clear cache and cookies? Search online for how to clear cache and cookies in Chrome (or Safari, Firefox — whichever you use).