This seems to be an issue with your TFS settings. Please find more information here. Please let us know if you can find a way to resolve this issue.
"Downloading the selected test run failed. Invalid cast from 'System.String' to 'Microsoft.VisualStudio.TestTools.Common.ITestElement'."
Do you have any suggestion?
Yes, this is possible. You can find information on how to do this in the Sending Results via E-Mail help topic.
Or is it possible to send and email after test from TestExecute?
The topic I mentioned contains information on how to send results via e-mail from a script or a keyword test. You can add a new test item to the end of your test project and send results of the test execution from this test item.
Yes we can send test results by opening test complete; but i want to send email automatically after test complete or fail. Because we are triggering TestExecute with Team Build at nightly builds.