The default 2018 Esri Business Analyst starting map documents do not open in ArcGIS Desktop 10.5.1.

Workaround

1. Open a blank map document.
2. Click Add Data and navigate to F:\ArcGIS\Business Analyst\US_2018\Data to add the layer files from each of the three default starting map documents, or pick and select to create a new default starting map document.
3. When adding basemap layer files, select Yes if prompted to enable Maplex.

Steps to Reproduce