Welcome to the forum.
First, please tell us a lot more about what you have and what you are doing.
Tell us about your Project Preset.
Tell us about your Assets that you are trying to place onto the Timeline.
Tell us the exact operation that you are employing to get them there.
A screen-cap of your Project's Timeline and the Project Panel might go a long way to telling us what you have.
Most often this is operator error, trying to drag clips to a sequence pane, with no sequence open in the pane.