I have read check documentation: https://checkstyle.org/checks/misc/todocomment.html
I have downloaded the latest cli from: https://checkstyle.org/cmdline.html#Download_and_Run
I have executed the cli and showed it below, as cli describes the problem better than 1,000 words

https://google.github.io/styleguide/javaguide.html#s4.8.6.2-todo-comments

A TODO comment begins with the word TODO in all caps, a following colon

Config:

<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
    "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
    "https://checkstyle.org/dtds/configuration_1_3.dtd">

<module name="Checker">
  <module name="TreeWalker">
    <module name="TodoComment"/>
  </module>
</module>

Code:

/** some javadoc. */
public class Test {
  void myFunc(int i) {
    // todo: implementing  // EXPECTED VIOLATION
  }

  void myFunc1(int i) {
    // Todo: implementing  // EXPECTED VIOLATION
  }
  
  void myFunc2(int i) {
    // toDo: implementing  // EXPECTED VIOLATION
  }

  void myFunc3(int i) {
    // TODO: implementing  // ok 
  }
}

CLI:

$ java -jar checkstyle-10.26.1-all.jar -c try_config.xml Test.java 
Starting audit...
[ERROR] /mnt/5D92528E6B945467/test/testing/Test.java:16:7: Comment matches to-do format 'TODO:'. [TodoComment]
Audit done.
Checkstyle ends with 1 errors.

As a part of the fix, we need to add TodoCheck to the google config and validate TODO is all in caps.