webxml attribute is required (or pre-existing WEB-INF/web.xml if executing in update mode)

If you are adding pom.xml in your existing web project, then it could be looking for web.xml.

To add web.xml add following line of code in pom.xml, make sure the path of web.xml  is correct.

<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-war-plugin</artifactId>
<configuration>
<webXml>WebContent\WEB-INF\web.xml</webXml>
</configuration>
</plugin>