Site is not configured for Claims Forms Authentication

Hi All

I have one custom login page for my Sharepoint 2013 site where i am trying to use

SPClaimsUtility.AuthenticateFormsUser(uri, strUserName, strPwd);

But i am getting this exception ---> Site is not configured for Claims Forms Authentication

I checked ULS long and there it is --> Not in claims forms auth for url 'https://domain/sitecollection/'.

Web.config enteries are 

------------------------------------

<authentication mode="Forms">
      <forms loginUrl="/_layouts/TarsForwardLogin.aspx" />
    </authentication>

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<membership defaultProvider="i">
      <providers>
        <add name="i" type="Microsoft.SharePoint.Administration.Claims.SPClaimsAuthMembershipProvider, Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
        <add name="membership" type="Microsoft.Office.Server.Security.LdapMembershipProvider, Microsoft.Office.Server, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" connectionUsername="cn=sharepointadmin,ou=GSP,ou=Applications,o=TOYOTA" connectionPassword="spad1n2" server="10.100.5.119" port="389" useSSL="false" userDNAttribute="entryDN" userNameAttribute="cn" userContainer="o=toyota" userObjectClass="Inetorgperson" userFilter="(ObjectClass=Inetorgperson)" scope="Subtree" otherRequiredUserAttributes="sn,givenname,cn,fullname,language,mail" enablePasswordReset="false" enablePasswordRetrieval="false" passwordFormat="Clear" requiresQuestionAndAnswer="false" requiresUniqueEmail="false" />
      </providers>
    </membership>
    <roleManager defaultProvider="c" enabled="true" cacheRolesInCookie="false">
      <providers>
        <add name="c" type="Microsoft.SharePoint.Administration.Claims.SPClaimsAuthRoleProvider, Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
 <add name="rolemanager" connectionUsername="cn=sharepointadmin,ou=GSP,ou=Applications,o=TOYOTA" connectionPassword="spad1n2" type="Microsoft.Office.Server.Security.LDAPRoleProvider, Microsoft.Office.Server, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71E9BCE111E9429C" server="10.100.5.119" port="389" useSSL="false" groupContainer="ou=groups,ou=GSP,ou=Applications,o=toyota" groupNameAttribute="cn" groupMemberAttribute="member" userContainer="o=toyota" userNameAttribute="cn" userDNAttribute="entryDN" dnAttribute="entryDN" groupFilter="(ObjectClass=groupOfNames)" userFilter="(ObjectClass=Inetorgperson)" scope="Subtree" />     
</providers>
    </roleManager>

Can someone please help/Guide

Manoj Gangwar | Sharepoint MCPD | Sharepoint MCTS |