Why is there no outline in 3d View when I select an object? If I select an object, the outline does not show up in 3d View. I mean there is no highlight around the object so that you can recognize what's actually selected. How can i fixed it?
Go to the Filter Menu and enable Selectable in the Restriction Toggles list:

See: https://docs.blender.org/manual/en/latest/editors/outliner.html#filter