We present the optimized atomic and electronic structures of a defective Ca2N bilayer with a monovacancy. Using first-principles calculations, we investigate the effect of a vacancy on the electronic properties of interlayer anionic electrons (IAEs) and surface anionic electrons (SAEs) in defect-free and defective structures. Electron clouds are not uniformly distributed in the interlayer region of the defective Ca2N bilayer. In the absence of Ca atoms, electrons in a specific place of the interlayer decrease by about 40% compared with a defect-free bilayer. Besides, we obtain simulated images for scanning tunneling microscopy (STM) to explain the SAE states of the 2D electride bilayer. We find that the STM simulation can determine which atom is missing from the STM images. We hope that our study will enable one to understand how defects affect the properties of electrides and how the defects can be used to modify them.