For ArcGIS Enterprise 10.7 product: portal
Error: The index service seems to be unavailable. Please check the index service
The Index Service has a safety feature that puts your index in read only mode if you have less then 10% disk space left.
Errors include can't add users cant change password cant delete users etc.
Resolution
Please check the log file to confirm that this is your problem.
C:/Program Files/ArcGIS/Portal/framework/runtime/ds/usr/logs/[Your Server Name]/elasticlog
It this is the problem make more space on the hard drive.
I have a call logged with Esri.inc to find out if we can change the % to a fixed size because if you use a 1Tb disk then you need 100GB of free space all times.
Hope this helps