In Visual Effect column in Place Actor, drag Planar Reflection or/Box/Sphere Reflection capture
Need to tick Planar Reflection mode on in Project Setting → Rendering
Another way, not by FX column, type Scene Capture Cube, drag in scene on place on the plane (it’s a camera)
→
Material → Cube Render Target (rename as CR Mirror)
Open the cube target, Increase thesiIze X resolution to 1024
Select the plane and the camera in the scene, drag the CR Mirror material in
Create a material, name as Camera Mirror, open its event graph, set as follow:
Now the material can be used, but its resolution will be lower in play mode. To solve it:
select the SceneCapture Cube camera, click bluprint → Convert Selected to Blueprint class (rename as Mirror_BP)
Open the Mirror BP event graph, delete the original nodes, drag the scene component from Component in graph, set the graph as follow: