The configuration section 'system.web.extensions' cannot be read because it is missing a section declaration on server xxx

set-WebConfigurationProperty -filter /system.WebServer/security/authentication/digestAuthentication -location $sitedir -PSPath "IIS:\" -name enabled -value false -ErrorVariable err -ErrorAction SilentlyContinue

 

when I run above command, got the error:

Filename: \\?\E:\wwwroot\farmTen1.xxx\web.config Line number: 1034 Error: The configuration section 'system.web.extensions' cannot be read because it is missing a section declaration on server xxx

 

anybody can help me? thanks in advance.