we could not complete your purchase(null)can't be installed on macintosh HD because OS X version 10.13 or later is required

Until you update the OS on your Mac, you cannot download that app.

ASC - Late 2012 Mac mini, macOS 10.15.5; Watch, series 5 GPS/cellular, watchOS 6.2; iPad Air 2, ipadOS 13.5.1; iPhone 8+, iOS 13.5.1; TV 4th Gen, tvOS 13.4.6;  HomePods, audioOS 13.4.6 and Airport Express w/ Airplay 2