As nucleus is positively charged and electron is negatively charged in an atom , both of them having opposite charges then why they did not attract each other.

The exact question that you posed today was addressed by several scientists. Since all physics theories are based on actual data, I chose the term "tried" to emphasize that while a physics theory may not satisfy all of the experimental facts, it does satisfy the majority of them.
The subatomic particles located outside the nucleus are known as electrons. Let's now examine various hypotheses that sought to address this issue. Bohr explained why electrons don't fall into nuclei in his model.
According to Niels Bohr's concept, electrons may only circle steadily and without emitting radiation within a specific, predetermined range of distances from the nucleus. Bohr referred to these orbits as "stationary orbits." These orbits, which are also known as energy levels or energy shells, are connected to certain energies. As needed by classical electromagnetics, radiation and energy loss are not produced by the electron's acceleration in these orbits.
It was evident by the 1920s that an extremely small item like an electron could not be regarded as a classical particle with defined location and velocity. The most we can do is estimate the likelihood that it will appear at any given location in space. As an electron advances toward the nucleus's attractive field, its potential energy decreases, and eventually approaches negative infinity. The loss of potential energy is made up for by an increase in the kinetic energy of the electron, which determines its momentum and effective velocity, because the total energy is constant (a hydrogen atom sitting calmly by itself will not lose or gain energy).
Therefore, when the electron gets closer to the tiny amount of space inhabited by the nucleus, its kinetic energy (momentum and velocity) shoots up toward positive infinity and its potential energy plunges down toward minus infinity. The theory informs us that the decline in potential energy is just twice as much as the kinetic energy and that the electron dances at an average distance that corresponds to the Bohr radius as a compromise since this "war of the infinities" cannot be won by either side.