Hi,
I ran into the same issue. To get a successful import, I included only the following fields in my "Input Occupant Table:" Known As, Email, Site ID, Contact Phone, Job Title, Org_Level_1, Org_Level_2, Start Date.
Furthermore, I left the "Unit Identifier (Input Occupant Table)" blank. These changes resulted in a successful import.
I was also attempting to assign the occupants to specific Units and Levels based on their respective IDs located in the Indoors attribute tables. However, those attempts resulted in repeated failures. I believe that functionality may be reserved for Indoor Space Planner in ArcGIS Online.