System.IdentityModel.Tokens.SecurityTokenValidationException: The token being parsed does not have an issuer.
   at Microsoft.IdentityModel.S2S.Tokens.JsonWebSecurityTokenHandler.ReadTokenCore(String token, Boolean isActorToken)
   at Microsoft.IdentityModel.S2S.Tokens.JsonWebSecurityTokenHandler.ReadToken(String token)

WebHeaderCollection headers = WebOperationContext.Current.IncomingRequest.Headers;

string contextTokenSting = headers["S2SSecurityContext"];

SharePointContextToken contextToken = TokenHelper.ReadAndValidateContextToken(contextTokenSting, sharepointUrl.Authority);

string accessToken = TokenHelper.GetAccessToken(contextToken, sharepointUrl.Authority).AccessToken;

ClientContext clientContext = TokenHelper.GetClientContextWithAccessToken(sharepointUrl.ToString(), accessToken);

Web web = clientContext.Web;
File file = web.GetFileByServerRelativeUrl(serverrelative);
clientContext.Load(file);
clientContext.ExecuteQuery();

string realm = TokenHelper.GetRealmFromTargetUrl(sharepointUrl);

string accessToken = TokenHelper.GetAppOnlyAccessToken(TokenHelper.SharePointPrincipal, sharepointUrl.Authority, realm).AccessToken;

using (var clientContext = TokenHelper.GetClientContextWithAccessToken(sharepointUrl.ToString(), accessToken))
{
     //Do your operation
}