added troubleshooting section for azure VNET

Author: sachin-sumologic

docs/integrations/microsoft-azure/azure-virtual-network.md

@@ -291,6 +291,10 @@ import AppUninstall from '../../reuse/apps/app-uninstall.md';
 
 ## Troubleshooting
 
+### App installation failed - Content Error - Invalid Field Extraction Rule.
+**Details**: This error is encountered if the app which is being installed, is trying to create an FER by a name which already exist as an FER in the org. This FER could have been created manually or through some other app.
+**Steps to resolve**: Rename or delete the existing FER and try reinstalling the app
+
 ### HTTP Logs and Metrics Source used by Azure Functions
 
 To troubleshoot metrics collection, follow the instructions in [Collect Metrics from Azure Monitor > Troubleshooting metrics collection](/docs/send-data/collect-from-other-data-sources/azure-monitoring/collect-metrics-azure-monitor/#troubleshooting-metrics-collection).