Had the same error message, it turned out admin module has more dependencies and some were missing (in my case pandas). Try putting:
import arcgis.gis.admin
in standalone script, and see what happens (I got as below, and installing pandas solved the problem)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:\Miniconda3\lib\site-packages\arcgis\gis\admin\__init__.py", line 9, in <module>
from .portaladmin import PortalAdminManager
File "C:\Miniconda3\lib\site-packages\arcgis\gis\admin\portaladmin.py", line 10, in <module>
from ...apps.tracker._location_tracking import LocationTrackingManager
File "C:\Miniconda3\lib\site-packages\arcgis\apps\__init__.py", line 4, in <module>
from . import survey123
File "C:\Miniconda3\lib\site-packages\arcgis\apps\survey123\__init__.py", line 3, in <module>
from ._survey import SurveyManager, Survey
File "C:\Miniconda3\lib\site-packages\arcgis\apps\survey123\_survey.py", line 10, in <module>
import pandas as pd
ModuleNotFoundError: No module named 'pandas'