cvc-id.3: A field of identity constraint 'web-app-servlet-name-uniqueness' matched element 'web-app', but this element does not have a simple type.

 The above error will occur in your web.xml while writing Servlet Code or Spring WebMVC code. 

The Error should be

 cvc-id.3: A field of identity constraint 'web-app-servlet-name-uniqueness' matched element 'web-app', but this element does not have a simple type.

you should change the xmlns part in the below image highlighted. This is causing the error.

Change it from 

xmlns = "http://java.sun.com/xml/ns/j2ee" 

TO

xmlns = "http://Java.sun.com/xml/ns/j2ee" 

OR

xmlns = "http://JAVA.sun.com/xml/ns/j2ee"