Google Cardboard Icon not displaying Samsung Android Phone(s) in Youtube App

You could try these methods to resolve the issue:

1. Logging out of Google account in the Youtube App

   
   
   
   
   
   • Some users suggest a workaround involving logging out of the Youtube App to resolve the cardboard icon.
   
   

Here is one user's explanation:

I logout my google account in the youtube app, the cardboard icon
appears and can view the video now in 360°

2. Fix resolution of phone.

   
   
   
   
   
   • If you happen to have changed the DPI on your phone, try reverting back to regular resolution.
   
   

3. Uninstall Youtube Updates

• Since the cardboard icon used to work fine in older builds of Youtube, temporarily downgrading the Youtube App in the meantime until the issue is resolved.

  
  
  

  Many issues feel the issue has to do with the Youtube App itself :

Only appears to be a problem with YouTube. Cardboard apps and other
videos I have tried work fine.

Also;

The cardboard icon disappeared after last update and it was working
perfectly in previous version.The cardboard VR apps are still working.

4. Other issues: Incompatibility

   
   
   
   
   
   • Your device may not be marked as compatible with Cardboard. If your device isn't compatible then that explains why you don't see that option.
   
   

Hope this helps

References

1. YouTube Help Forum: No cardboard icon displayed in updated Android YouTube app


2. No Cardboard icon in YouTube app