I have added the 2D box collider to the floor, 2D box collider and 2D rigid body to the player. But the player still fall through the floor.

Questions & Answers
1

The problem is recorded here: https://www.loom.com/share/75a374169a2c446186c5044f7c65dd36

2

The floor collider should not be of type isTrigger, uncheck that.