Did you find all 5 Moley Plants first? You need these before you can get the mandrake root. You'll find them at:
1. The first plant is right near Ernie in the middle of Hagrid’s pumpkin patch.
2. There is a Moley plant beside an upright stone on the path near Hagrid’s
Hut.
3. This one is near the stone circle on the path leading to and from Hagrid’s
Hut.
4. In the stone circle near the entrance to the covered bridge.
5. By the path leading from the stone circle to the owelry, at the base of the
hill.