Hidden away in a note in Apple support is the announcement that Apple’s macOS will no longer support Aperture after the current Mojave (10.14) version. Apple discontinued the development of Aperture in 2014.
The support note goes on to provide a series of solutions for users still using Aperture. These include migrating images and associated libraries to Apple’s Photos app or to Adobe Lightroom. Commencing version 5.7, the latter has a specific tool to perform this migration.