In June 2014, Apple announced the discontinuation of development of Aperture. Since that time, Apple has released five major macOS updates. For technical reasons, Aperture will not run in future versions of macOS after macOS Mojave. To continue working with your Aperture photo libraries, you must migrate them to the Photos app included with macOS, or migrate them to another application.
Archived Aperture support articles
Final Aperture user guide
Aperture 3.5 User Guide (PDF)