The latest jammr 1.3.0 release requires macOS 10.13 High Sierra or later. However, you can download the previous jammr 1.2.8 release here
There are instructions for disabling the update checker here
if you don't want to be prompted to update jammr each time you launch it.
jammr 1.2.8 currently works fine but at some point in the future it will no longer function as jammr evolves. Only the latest stable release is officially supported.