Insights for ArcGIS does not require licensing through the License Manager.
To license Insights for ArcGIS, import the .json license file downloaded from My Esri directly into the Enterprise portal. You import the license file into Portal in the Portal Administrator Directory under System > Licenses > Import Entitlements.
Here is the documentation on configuring Insights for ArcGIS licenses for ArcGIS Enterprise 10.5.1: https://enterprise.arcgis.com/en/portal/10.5/administer/windows/configure-insights-licensing.htm
Hope this helps,