Could not load Kentor.AuthServices. A strongly-named assembly is required

I want to use Kentor.AuthServices to authenticate in my web app using Okta. I followed the article "How to use KentorIT AuthServices with Okta".

Because the application would throw an error:

The signing algorithm http://www.w3.org/2001/04/xmldsig-more#rsa-sha256 is weaker than the minimum accepted http://www.w3.org/2000/09/xmldsig#rsa-sha1.

That's why I added in global.asax following line:

protected void Application_Start()
{
    Kentor.AuthServices.Configuration.Options.GlobalEnableSha256XmlSignatures();

This line causes following error:

Could not load file or assembly 'Kentor.AuthServices, Version=0.21.1.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. A strongly-named assembly is required. (Exception from HRESULT: 0x80131044)

How to solve this?