User tries to enable Gateway insight on a VPN Gateway in NetScaler MAS. Creates a policy but when they click OK to confirm creation they get he following error:
Error in retrieving appflow.vpn_error_counts. event is not defined

Complete the following steps to resolve this issue:

1. Go to Infrastructure node, then click the NetScaler that you want to monitor. Right-click Enable/Disable Insight option.
2. Under Application List, in the View list tap, select VPN.
3. Select the Virtual server for which you want to enable AppFlow. In the Action list, click Enable AppFlow.
4. Select the HTTP check-box.
5. Once the correct settings are applied data is displayed.
6. After this re-enter the policy and tick ICA and TCP.

Citrix ADM version 12.1 build 49.37 and above has self-service diagnostics for Analytics which is used to identify the license and configuration issues on the managed instances which could lead to no data being captured. The self-service diagnostics helps you to identify and troubleshoot the issues faster. For more information refer to Citrix Documentation - Self-service diagnostics for Analytics.