Etabs doesn't calculate and showing the Moment capacity of Beam. however, etabs just calculates the required amount of rebars (longitudinal, transversal) based on the ultimate moment.
Most of the design software calculate forces acting on the section size selected and based on Moment, shear force and Torsion acting on the beam the reinforcement is calculate. Whereas you are trying to find out moment resisting capacity of a given section.
Based on the longitudinal reinforcement ratio and classic design methodology for RC beams according to ACI 318-19 you can calculate the moment capacity.