GWT error even on official tutorials: Check that your module inherits 'com.google.gwt.core.Core' either directly or indirectly - gwt

Question

I am learning GWT and was trying to run this tutorial by Vogella and also the official GWT tutorial.
Using the Eclipse GWT plugin 3.0 on Windows 10 and JDK 11
I marked the Maven Project checkbox.
When I right-click and Run As
Turning off precompile in incremental mode.
Super Dev Mode starting up
workDir: C:\Users\My\AppData\Local\Temp\gwt-codeserver-8137229043727681777.tmp
2021-04-29 12:03:13.494:INFO::main: Logging initialized #718ms
Loading Java files in de.vogella.gwt.helloworld.De_vogella_gwt_helloworld.
[ERROR] Hint: Check that your module inherits 'com.google.gwt.core.Core' either directly or indirectly (most often by inheriting module 'com.google.gwt.user.User')
I see that it is inheriting User
<?xml version="1.0" encoding="UTF-8"?>
<!--
When updating your version of GWT, you should also update this DTD reference,
so that your app can take advantage of the latest GWT module capabilities.
-->
<!DOCTYPE module PUBLIC "-//Google Inc.//DTD Google Web Toolkit 2.8.1//EN"
"http://www.gwtproject.org/doctype/2.8.1/gwt-module.dtd">
<module rename-to='de_vogella_gwt_helloworld'>
<!-- Inherit the core Web Toolkit stuff. -->
<inherits name='com.google.gwt.user.User'/>
<!-- Inherit the default GWT style sheet. You can change -->
<!-- the theme of your GWT application by uncommenting -->
<!-- any one of the following lines. -->
<inherits name='com.google.gwt.user.theme.clean.Clean'/>
<!-- <inherits name='com.google.gwt.user.theme.standard.Standard'/> -->
<!-- <inherits name='com.google.gwt.user.theme.chrome.Chrome'/> -->
<!-- <inherits name='com.google.gwt.user.theme.dark.Dark'/> -->
<!-- Other module inherits -->
<!-- Specify the app entry point class. -->
<entry-point class='de.vogella.gwt.helloworld.client.De_vogella_gwt_helloworld'/>
<!-- Specify the paths for translatable code -->
<source path='client'/>
<source path='shared'/>
<!-- allow Super Dev Mode -->
<add-linker name="xsiframe"/>
</module>
This SO question was not relevant to my issue.

Answer 1

Almost certainly you are either missing gwt-user.jar from your classpath, or somehow the gwt-user.jar version doesn't match the gwt-dev.jar, which will cause problems. Every module automatically inherits com.google.gwt.core.Core (despite the error message), and as your .gwt.xml shows, you already have User added.
--
Additionally, from your linked SO post, do check the comments - there are some better, more modern tutorials listed.