Why is General Theory of Relativity not applicable in the singularity of Black Hole?
The singularity of a Schwarzschild black hole is not part of the manifold but instead a point in time. General Relativity deals with manifolds and thus isn't applicable in the singularity.
You can find more background here: Is a singularity a single point?
A singularity in GR is like a piece that has been cut out of the manifold. It's not a point or point-set at all.