You placed the panel and cover parts; however, they are not in their final positions. You use assembly constraints to locate the parts with respect to one another. Typically, you need three constraints to position a part.
Currently, the panel and cover parts are unconstrained and are free to move in 3D space. Click and drag the cover part. Notice that the part moves with your cursor, and the part remains at whatever location you release the mouse.
Click in the following image to play an animation that shows dragging the cover part within the assembly.