Why is photoelectric effect not possible at all the frequencies?

Hint: Whenever the beam of electrons strike the metal surface then electrons are emitted out of the metal surface but these electrons are not emitted out as soon as the electron strike in fact it depend upon the vivid number of factors one of the threshold frequency use this concept to approach the solution.

Step by step solution:
Threshold frequency: threshold frequency is the minimum frequency of the incident radiation below which no photoelectric effect will occur irrespective of the intensity of the incident radiation.
Photoelectric effect: photoelectric effect is the emission of the electrons when a ray of light coming from the source i.e. sun (electromagnetic radiation coming from the sun) is incident on the material such as a metal piece then electrons are emitted out. And these emitted electrons are known as photoelectrons.
So, the threshold frequency is the frequency of light which carries enough energy to dislodge electrons from an atom i.e. release an electron from the metal which contains electrons in the rest position, every metal contains the electrons.
So, if the frequency of incident radiation is less than threshold frequency of the metal because of this no electrons have released from the atom as electron gas contains no kinetic energy or not enough kinetic energy to dislodge an electron from the atom or metal piece.
As the electron does not gain enough velocity so they do not gain enough kinetic energy. As kinetic energy is half times the mass multiplied by square of the velocity of electrons i.e. $K.E = \dfrac{1}{2}m{v^2}$

Hence, we can say that photoelectric emission only occurs when the frequency of the incident radiation is greater than the threshold frequency or we can say that photoelectric emission does not occur at all frequencies.

Note: The main reason behind the emission of the electrons from the metal surface when a wave of light hits the metal surface is due to the dual behavior of light since light can act both wave and particle, so this depicts that light is composed of photons or energy quanta. These photons when hit the surface transfer energy to the surface electrons and thus making the emission possible.