Silverlight is no longer supported in Safari 12.
This article posted on apple site informs:
is there any way to enable the Silverlight Plug-In in Safari 12?
No, there is no way to make the current version of the Silverlight plugin work with Safari 12.
As of today, no actively supported web browser on macOS can run the current Silverlight plugin (neither Safari, Chrome nor Firefox, see below for more information). The culprit is the technology the plugin is based on: NPAPI, which has been deprecated in all major browsers because of security concerns, the latest of them being Safari with version 12.
This means that you must use an unsupported browser, with all the risks that entails.
To continue using Silverlight on macOS, your "safest" bet would be Firefox 52 ESR for accessing BI Office. You can download it from Mozilla's release repository.
Firefox 52 ESR has been replaced with Firefox 60 ESR, but its latest version is somewhat recent, released on June 25 2018.