Created an external list SharePoint 2016 using business data connectivity services, configured secure store target application and created external list in SharePoint.
The external list displayed this error message with a correlation ID.

“Unable to render the data. If the problem persists, contact your web server administrator.”

ULS Log viewer found this message in the logs:

Error while executing web part: Microsoft.BusinessData.Infrastructure.BdcException: The shim execution failed unexpectedly – Unable to obtain the application proxy for the context.. —> Microsoft.Office.SecureStoreService.Server.SecureStoreServiceException: Unable to obtain the application proxy for the context.

Solution:

The web application is not associated with the secure store service application.

• Go to SharePoint Central Administration site
• Application management >> Click on Configure service application associations under Service Applications.
• select the web application in which your site exist, Check the “Application proxy group” column >> Make sure the BDC and secure store service applications check boxes checked.