Answer:
The answer is: The bed is so hard her body leaves no dent.
Step-by-step explanation:
Impression used here means leaving a dent. This is the depression that shows when we sit or lie on a couch or bed.
When the author tries to sleep in the above text, due to the way the bed was made (The bed had been made high in the center and sloping on either side) the author couldn't make an impression on the bed.
Therefore, impression in this case means: "The bed is so hard her body leaves no dent."