Streamus had some issues with Youtube and they were unable to come to an agreement. So it was removed from the Chrome webstore and (apparently) also from the Opera one. It had his API key removed so old Streamus extensions will not be able to add new songs to their playlist. However there are still ways to install Streamus with full functionality. Refer to the streamus reddit if you want to learn how to keep using it. Otherwise you are out of luck.