That generally happens because there is a glare on the image from the lights in the machine lid. (And it’s hard to get a complete scan when there are big solid black filled areas.)
You can use the up and down arrow keys on the keyboard during the scanning step to improve what gets captured. Make sure the paper is completely flat. And also try shifting the paper slightly to the left or right to get rid of the glare reflection.
(Instructions on how to use the Trace tool are laid out at the tutorial below.)