Backup REST services failing

[LefterisKoumis]

I am trying to schedule a backup of the REST services. 

So, I am running the script with Admin credentials on CMD window:

C:\Python27\ArcGIS10.8\python.exe "D:\ArcGIS\Server\tools\admin\backup.py" -u XXXX -p XXXXX -s https://XXXXX/rest/services -f "D:\GIS_Data\backup

but I get this error

I double checked the credentials and the server url and are correct.

I am aware that I can back up the REST directories by using the 

https://XXXXX/admin/exportSite 

and just define the destination, but I need to schedule the task.

Ideas?

Thanks.

[MichaelVolz]

Is this a standalone ArcGIS Server or part of a Portal?

Did you ever try this script in an earlier version of ArcGIS?

[LefterisKoumis]

Standalone.

No, this the first time I am trying this script from:

https://enterprise.arcgis.com/en/server/latest/administer/windows/backup-utility.htm