java.lang.AssertionError: found value shows as null

I wanted check whether the text field value is same or not to the expected value which i mentioned on the code

This is the text field which i needed the value

input type="text" value="sadas" class="mdl-textfield__input" id="last_name" name="last_name" placeholder="Enter last name"

Error what i got,

TestNG error message shows as follows, java.lang.AssertionError: expected [6234] but found [] and nothing printed for the console as well

I have tried with "Assert.assertTrue(lastName.equals("lastName : 6234"));" too

@Test
    public void tc001() {       
    driver.get(baseUrl);
    driver.findElement(By.xpath("//input[@name='email']")).click();
    driver.findElement(By.xpath("//input[@name='email']")).clear(); driver.findElement(By.xpath("//input[@name='email']")).sendKeys("x@gmail.com");
    driver.findElement(By.xpath("//input[@name='password']")).clear();driver.findElement(By.xpath("//input[@name='password']")).sendKeys("123456");
    driver.findElement(By.xpath("(.//*[normalize-space(text()) and  normalize-space(.)='Forgot Your Password?']) [1]/preceding::button[1]")).click();
    driver.findElement(By.linkText("Nadee")).click();
    driver.findElement(By.linkText("Profile")).click();
    String lastName = driver.findElement(By.xpath("//input[@name='last_name']")).getText();
    Assert.assertEquals(lastName ,"6234");
    System.out.println(lastName);   
    driver.findElement(By.linkText("Log Out")).click();
    }

How can i fix this? And why i'm getting this error when expected value same to the found?(when manually checking the system)

try to add wait before getting value of lastname field. You are clicking a link before getting last name value, there is a possibility that page was not fully loaded.

driver.findElement(By.linkText("Profile")).click();
WebDriverWait wait = new WebDriverWait(driver, 10);
wait.until(ExpectedConditions.visibilityOfElementLocated(By.xpath("//input[@name='last_name']")));
String lastName = driver.findElement(By.xpath("//input[@name='last_name']")).getAttribute("value");
system.out.println("Last Name value is: "+ lastName); // check the output on console too
Assert.assertEquals(lastName ,"6234");