Herein, the thermoelectric performance of (Formula presented.) ternary layered dichalcogenides is evaluated by applying ab initio density functional theory calculations combined with Boltzmann's transport equation. A novel approach to design the intrinsic structural defects via Se-anion vacancies in unit cell has been developed. Two kinds of (Formula presented.) -vacancy defects in host (Formula presented.) crystal lattice are engineered: the single vacancy defect induced intrinsically in the unit cell ((Formula presented.)) and in the supercell lattice ((Formula presented.)). It is found that the electrical transport properties and thermoelectric efficiency of this semiconductor could be significantly altered by introducing Se-vacancy states into crystalline structure. In addition, simulation shows that inclusion of Se-vacancy defects significantly improves the thermoelectric efficiency as well as the thermoelectric power factor and figure of merit ((Formula presented.)) values of this compound. Additionally, the thermoelectric performance of (Formula presented.) is estimated by means of the electronic fitness function calculations in the valence and conduction edges. The results demonstrate that (Formula presented.) with introduced (Formula presented.) -vacancies may be a perspective material for thermoelectric applications.