Question

What does the negative value of $E_{cell}^0$ indicate?

Answer 1

The electromotive force (also known as cell voltage or cell potential) between two half-cells is denoted $E_{cell}^0$. The stronger the pushing force of electrons through the system, the more probable the reaction will proceed (more spontaneous). The voltage of an $E_{cell}^0$ is measured in volts (V).

Complete answer:
The energy associated with the separation/transfer of charge is the property of potential, E. A cell potential's arithmetic sign determines the spontaneity of the cell reaction, with positive values indicating spontaneous reactions and negative values indicating nonspontaneous reactions (spontaneous in the reverse direction).
If the value is positive, the oxidation-reduction reaction is spontaneous. That is, without the assistance of a third party. The cathode undergoes reduction, while the anode undergoes oxidation. If the value is negative, it signifies that only the opposite reaction is spontaneous. It refers to the cathode's oxidation and the anode's reduction.
The change in Gibbs free energy in a galvanic cell, where a spontaneous redox reaction drives the cell to produce an electric potential, must be negative. The cell potential, which is positive when electrons flow freely through the electrochemical cell, is the polar opposite of this.
$\Delta G_{cell}^0 = - nFE_{cell}^0$.
So, the final answer will be that,
Negative $E_{cell}^0$ value means, ${\Delta _r}{G^0}$ will be positive and hence the cell will not work.

Note:
The Gibbs Free Energy is just a way of determining whether a chemical reaction will occur spontaneously or not. The Gibbs free energy determines how much energy is available in a system to perform work. Only systems with constant temperature and pressure can use Gibbs free energy.