Thanks for your reply. Please see attached for the GWT lib specified in the java build path in Eclipse. I tried to compile it with JDK 8 and it didn't work either.
The same project configuration in Eclipse worked in my old laptop, but not in my new laptop. The Eclipse in the new laptop is a copy of the Eclipse installed in the old laptop.
In addition, I also tried to copy the "gwt-dev.jar" from the old laptop to the new one, but the compilation still gave me the following error.
Error: Could not find or load main class com.google.gwt.dev.Compiler
The project was compiled by using GWT->Compile option provided by the GWT Plugin installed in Eclipse.
Version: 2020-06 (4.16.0)
GWT Plugin version: 3.0
Your help is greatly appreciated!