A negative potential energy is possible. Explain

Yes, negative potential energy is possible.
When a stone is thrown into the air, it receives positive potential energy (m g h) when the earth is taken into account as the origin.
Planets have negative potential energy because they are in the Sun's gravitational field.
Negative potential energy simply indicates that the planet is constrained and needs an equal amount of external energy to break free from the Sun's gravitational field.
In quantum physics, the same holds true for electrons bound to atoms.