Hi , I am using the sample web ui testing Healthcare. Now I cannot find where this Username & Password store. Already click but nothing.
The answer is kind of hidden in your question.
A variable like Username refers to a chunk of memory, somewhere.
When you say:
String Username = "Batman"
you are telling the compiler where Username is and what is stored inside it.
When your variable looks like this:
The compiler is telling you it can’t find String Username anywhere. It’s not present in the current scope. So the compiler is asking you almost the same question as you are asking us - how’s that for karma? 
What I am asking whether it have shortcut to find this underline words. Usually in Android Studio when I click at the variable it will navigates to the directory. Now I am lost.
Listen carefully: Katalon is telling you it can’t find it. How will it create a shortcut to something it cannot find?
I think because it is underlined you think it should be a link. It’s not a link, it’s a warning.
Try searching for it: Menu > Action > Search.
Usually when I am in my twin engine Cessna, when I pull back I take off and start flying. But my car refuses to do that. When I pull back nothing happens (thankfully).
I am very appreciate your sarcasm especially helping the newbie. Now I already understand why the variable is underline. It is because the variables already set in variables tab then choosing the input values .
Great. Glad you found a solution.
I marked your response as the solution. 
Open the folder of “Sample Web UI Tests Project (Healthcare)” with Windows Explorer or Mac Finder or Linux Command line editors, you will find <projectDir>/Test Cases/Main Test Cases/TC1_Verify Successful Login.tc, which is a XML file like this.
<?xml version="1.0" encoding="UTF-8"?>
<TestCaseEntity>
....
<variable>
<defaultValue>'John Doe'</defaultValue>
<description></description>
<id>14d98e7b-4ac8-4e28-8ce7-394001d188f0</id>
<masked>false</masked>
<name>Username</name>
</variable>
<variable>
<defaultValue>'ThisIsNotAPassword'</defaultValue>
<description></description>
<id>a97344e7-5f9c-4936-92a2-d85e5463f3c9</id>
<masked>false</masked>
<name>Password</name>
</variable>
</TestCaseEntity>
Katalon Studio finds the values of “Username” and “Password” variables here. Katalon Studio knows this XML file and can happily consume it.
On the other hand, the underline for Username and Password is rendered by Eclipse-originated Editor, not really by Katalon Studio. The Editor seeks the definition of Username and Password in the Groovy source code, while the Editor knows nothing about the metadata in the TC1_Verify Successful Login.tc XML file. Therefore the Editor can not stop rendering underlines for Username and Password.
Once I was confused with this underline as well. I think this UI is poor (not as complete as Android Studio and IntelliJ IDEA). But it seems that the Katalon Team does not mind it much. I think that users should be aware of this poorness and accept it. It is not a big issue. Don’t mind it too much.