This is normally because gravity is enabled but collisions are not. Make sure that the collision button on the Blink 3D Viewer toolbar is down. If not in your project, go to the Player Beahvior and make sure the Enable Collisions property is set to True.
If you are using an Entry Point to enter the environment it may be that the position of the Entry Point is too low, initially placing the feet of the avatar below the ground level.