You need two things:
(1) A reflection probe that covers an appropriate area. The area should include both the mirror and everything around it for it to mirror.
(2) A reflective surface.
For (1): You can see reflection probes for your scene in the Graphics mod (F5), bottom of first tab. The default probe is located at XYZ 0, 0, 0 and covers an area (giving reflectivity to mirrors and reflects the things in that area) of XYZ 100 by 100 by 100. So any mirror that moves outside of that loses its reflectivity. You can place additional reflection probes around the map, which you need when you want mirrors to reflect different environments. Under the Settings tab, check "Realtime Reflection Probes" if you want dynamic reflections. For some reason, however, only the default reflection probe gives you real-time reflections, the custom ones only giving static/baked reflections.
For (2): To get a reflective surface, e.g. a mirror, go to the Material Editor of the object. Set its Metallic properties to max and its Glossiness to max.
I talked about it earlier here:
https://f95zone.to/threads/illusion-hs2-studio-neo-chara-maker-help-guides-thread.66252/post-9444753