The Living Atlas tab is missing for Default Administrator as well in ArcGIS Business Analyst Web App if the privilege 'Members can search for content outside the organization' is toggled off in ArcGIS Online.

Workaround

Toggle on the ‘Members can search for content outside the organization’ option from the Security settings under Organization in ArcGIS Online to enable the Living Atlas option in ArcGIS Business Analyst Web App.

Steps to Reproduce