Hello,
When the IIS Web Adaptor runs, it creates a new application pool in IIS called ArcGISWebAdaptorAppPool. The identity of this application pool needs permissions to the WebAdaptor.config file that is being referenced in the error message.
Could you please try the following:
1) Navigate to C:\inetpub\wwwroot\arcgis\ or the location referenced in the error message.
2) Right click the WebAdaptor.config file and choose Properties.
3) Click the Security tab.
4) Check whether the setup program has given the local user IIS APPPOOL\ArcGISWebAdaptorAppPool permissions to write to this file.
If it is not listed, we need to add it.
5) Click the Edit button.
6) Click Add.
7) Click the Locations button and select the name of the local machine.
😎 In the box at the bottom, enter 'IIS APPPOOL\ArcGISWebAdaptorAppPool' without the single quotes and click 'Check Names'.
9) Give 'Full Control' permissions to ArcGISWebAdaptorAppPool.
10) Click Apply and OK to all dialogs to close them.
Please open the Web Adaptor configuration page again and try registering your GIS Server machine.
Thank you
Derek