"ExpiresIn" not found

I added in unity 6000.0.31f .NET PayPal SDK and paypal CoreSDK libraries from NuGet for unity. I added to my script Using Paypal.api
the problem is “expiresIn” is not found and missing dirrective in the bellow script. Please help fix this issue

retryPolicy.ExecuteAsync(async () =>
{
    try
    {
        var oauthTokenCredential = new OAuthTokenCredential(config);
        var accessToken = oauthTokenCredential.GetAccessToken();
        cachedAccessToken = Encrypt(accessToken);
        DateTime dateTime = DateTime.UtcNow.AddSeconds(oauthTokenCredential.ExpiresIn);
        tokenExpirationTime = dateTime;
        Debug.Log("New access token retrieved: " + MaskAccessToken(accessToken));
        callback(accessToken);
    }
    catch (Exception ex)
    {
        Debug.LogError("Error retrieving access token: " + ex.Message);
        callback(null);
    }
});