In Atlassian Marketplace, the instructions on the installation tab for the stash backup client are incorrect:
https://marketplace.atlassian.com/plugins/com.atlassian.stash.backup.client#tabs-installation
These indicate that the backup client can be installed through the plug-in manager, which is not the case.
I was able to locate the correct instructions here:
https://confluence.atlassian.com/display/STASH/Data+recovery+and+backups