Just a background info on what is going on here:
Windows’ audio driver system is not good at running multi-audio stuff. However, multi-audio is what DAWs like Cubase usually need to do. So Steinberg invented ASIO, which is (to keep it simple) a replacement system for the Windows audio drivers for advanced audio usage.
However, that also means that Windows and Cubase run two different systems when accessing your soundcard/audio interface. Under certain circumstances one of these two systems uses the audio device exclusively, thus forbidding access to the other.
Or in simple words: YouTube works, Cubase doesn’t - or Cubase works and YouTube doesn’t.
If you change the settings so that the two driver systems (Windows and ASIO) can happily co-exist then your problem will be solved. This is what the replies above mine are trying to do.
There are audio devices, which do not allow this and their owners are just screwed but luckily these devices are getting scarce.