The additively manufactured (AM) parts have been introduced in many industrial fields, and their usage is growing. This increase of use forces the need for effective tools for damage evaluation. This study focuses on elastic guided waves. Their propagation was analyzed, and wave scattering was used for damage assessment. The samples under investigation are flat plates printed with poly(lactic acid) (PLA). A few plates were prepared with five infill densities. Solid printed parts use a lot of material, so the reduction of the infill density allows to save material required for manufacturing. On the other hand, the reduction of the infill density makes the structure similar to a sandwich composite, and such complex structure influences the guided wave propagation behavior. In this study, plates were prepared as healthy and with simulated damage. The simulated damage was a void introduced in the printing process. The guided waves in the plates were excited with surface-mounted piezoelectric transducers, while the sensing was realized with scanning laser Doppler vibrometer. Damage localization algorithms based on wave reflections were developed, and their performance was analyzed as a function of the infill density. The obtained results show a potential of guided waves-based techniques for the structural health monitoring of additively manufactured structures.