<tr role="row" class="odd">
<td class="sorting_1">Abrar</td>
<td>1571</td>
<td>Out</td>
<td>No</td>
<td>ALL</td>
<td>Deskflex</td>
<td>
<a class="btn" href="/override/OverUserOverRide/21801" style="font-size: 15px;"><span class="glyphicon glyphicon-user"></span></a>
</td>
</tr>
IWebElement t = driver.FindElement(By.XPath("//*table[@id='customerDatatable']"));
t.FindElement(By.CssSelector("a[href='/override/OverUserOverRide/21801']")).Click();
I have used this code but not working. Link
The problem is in the 1st element t locator, since it's not a valid one.
//*table[@id='customerDatatable']
You should use either * (for searching among all tags) or specific TagName (table in your case), so it could be:
//*[@id='customerDatatable']
or
//table[@id='customerDatatable']