SSL/TLS secure channel issue

See more:

C#

WCF

even after after adding

<system.net>
<settings>
<servicepointmanager checkcertificatename=""false"" checkcertificaterevocationlist=""false"">
<settings>
<system.net>;


this code in web config iam getting the error.

The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.

please tell what i have to change in web config