Youtube changed their embedding policies in August, resulting in some issues with our "Videos" tab: when trying to watch Youtube videos embedded in our app, users are now required to sign in to YouTube.
Our technical team are still investigating, but unfortunately, it does look like this is outside of our control.
In the meantime, try signing in to be able to watch all Youtube videos. Alternatively, you can filter all videos in the "Videos" tab by selecting "Memrise shorts" only, and this will hide YouTube videos. This way, you can still learn using the videos created by our internal team at Memrise.