If magnitude of vector cannot be negative how displacement can?

The MAGNITUDE cannot be negative but the vector itself can be.

Once you have chosen a direction for a quantity to be called POSITIVE then a vector of the same magnitude but the opposite direction must be equivalent to a vector of negative length in the positive direction.
ie a negative vector.

displacement is a vector so it's got a direction and a magnitude.

if you are asking about the magnitude of vector, in this case magnitude of displacement it's only got a magnitude and no direction.

But displacement itself has both

magnitude is the number. Displacement is the net change.

When dealing with vectors in 1 dimension (e.g left/right, or up/down)
***the sign (+ or -) tells you the direction***.

For displacement, the magnitude mean the distance between the start and end points.

E.g. Left/right motion with positive=right and negative=left.

Your displacement x = 5m.
The magnitude (distance between the start and finish points) is the magnitude of x which is |5m| = 5m.
You moved from left to right (because x is a positive value).

Displacement x = -5m.
The magnitude (distance between the start and finish points) is the magnitude of x which is |-5m| = 5m.
You moved from right to left (because x is a negative value).

The distance between 2 points is always a positive value. The magnitude of displacement (the distance) is therefore always a positive value.

A vector has magnitude and direction.

A negative displacement has a direction which is "backward", Positive and negative is part of the "direction" of the vector.

You can go north or south. It has a direction.
Taking one foot any where is one foot rod. It is a magnitude. You can point it north or south, then it is a vector.