You are probably passing wrong secret to server. Although secret must be hashed.
Try hashing the secret.
public static IEnumerable<ApiResource> GetApiResources()
{
return new List<ApiResource>
{
new ApiResource("default-api", "Default (all) API")
{
Description = "AllFunctionalityYouHaveInTheApplication",
ApiSecrets= {new Secret("secret").Sha256() }
}
};
}