I followed a tutorial for setting up a scene with light but got stucked at first point. I first create a plane and then I extrude it 90 degrees. Then I want to bevel the sharp edge to be round. I select the edge in edit mode and apply bevel. But i only works on one of the two planes. The tutorial was using 2.79 and I am using 2.8. Is there any reason why this not working?
There's a rather simple reason:
Your object's scale is messed up.
Your object has uneven scale, so the bevel is applied 400x times more on one side, than on the other.
Fix it by going to Object->Apply->Scale.
Then the bevel tool will work as you need it.
are you using Ctrl+B or the Bevel Modifier? could you add a screenshot? it would be very helpful
edit: I think your vertices might be disconnected, have you tried to start again with a new plane?
Check your right bottom info, without the bevel you should only have 2 faces and 6 vertices.