Let $n=4$. Label the squares $(a,b)$, $1\le a\le8$, $1\le b\le8$. Cut out the identical shapes $$(3,3),(4,3),(5,3),(6,3),(7,3),(5,2),(5,4)$$ and $$(2,6),(3,6),(4,6),(5,6),(6,6),(4,5),(4,7)$$ You'll find you can't cut out another copy of this shape.
Proof: any copy of this shape must have a row of five, horizontally or vertically. The row of five can't be along an edge of the square because there must be a square on either side of the row of five. No horizontal row, other than an edge, has five contiguous squares, once you have cut out the two shapes. The only columns with five contiguous squares, other than the edge columns, are columns 2 and 7, and those two locations for the 3rd shape are blocked by the missing squares at $(3,3)$ and $(6,6)$, respectively.