reason 9.5.4

The following products are NOT compatible with macOS Mojave:

Reason 9.5.4 and earlier

Reason Essentials 10 and earlier

Reason Limited 1.5.3 and earlier

https://forum.reasontalk.com/viewtopic.php?t=7508694&start=25

Propellerhead states Reason 10.2 and later are required for Mojave.

For details around why the earlier releases aren’t supported, you’ll want to ask the folks that are supporting it.

There are related discussions in the Propellerhead forum...