Did you get it right?
The rectangular jigsaw has three finished corners and one empty upper-right corner gap.

The Answer
The correct answer is: the final upper-right corner piece.
Here is why
- Three outer corners of the landscape puzzle are complete.
- The upper-right corner has one unmistakable corner-shaped gap.
- No loose visible piece has the required two straight outer edges.
Why people miss it
Loose pieces make the table look as though the answer is present somewhere, but none can complete that corner.
What answer did you get before checking?