Recently while I was trying to open a SharePoint 2016 site, it gave an error and the site does not open. The error comes as:
Server Error in ‘/’ Application.
The requested service, ‘http://localhost:32843/SecurityTokenServiceApplication/securitytoken.svc’ could not be activated. See the server’s diagnostic trace for more information.
New to Office 365 SharePoint Online? Get Office 365 Enterprise E3 Subscription & Try out all the features
Description:
An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it is originated in the code.
Exception Details:
System.ServiceModel.ServiceActivationException. The requested service….
It looks like below:

Also, you can read:
– SharePoint Online Delete all Items from a list using JavaScript Object model
– How to get alternate languages from language settings in SharePoint online using Rest API?
– Display Charts using Chart Js in SharePoint Online Office 365
Solution:
To resolve the issue I have just restarted the server and tried opening the SharePoint site and it worked fine for me.
Hope this will help you.