Same button is used for activate and deactivate. First time when the code run button clicked and "Deactivated" successfully.But on second time the code can't able to find the element. First case button is in second 'a' tag and then deactivated the button it is in first 'a' tag.
Case 1:
<td>
<a class="btn btn-success btn-sm fa fa-edit" href="/ClassDetails/Edit?ClassID=CLSS1012201800050">Edit </a>
<a class="btn btn-danger btn-sm fa fa-trash-o" href="/ClassDetails/Deactivate?ClassID=CLSS1012201800050">Deactivate</a>
</td>
Case 2:
<td>
<a class="btn btn-info btn-sm fa fa-check-square-o" href="/ClassDetails/Activate?ClassID=CLSS1012201800050">Activate</a>
</td>
Xpath is
WebElement deactivatebutton = driver.findElement(By.xpath("//*[@id='tblClassName']/tbody/tr[2]/td[4]/a[2]"));
//WebElement activatebutton = driver.findElement(By.xpath("//*[@id='tblClassName']/tbody/tr[2]/td[4]/a"));