Could not load file or assembly Recaptcha (Exception from HRESULT: 0x80131044)

I am running an old "webforms" .net 3.5 framework project written in (asp/vb). I've been instructed to implement Google reCaptcha on some of the forms since they are under attack at this point in time. Tried with the most recent version of reCaptcha and followed instructions on the following page,

https://developers.google.com/recaptcha/old/docs/aspnet

However, I'm getting the following error,

Could not load file or assembly 'Recaptcha, Version=1.0.2.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. A strongly-named assembly is required. (Exception from HRESULT: 0x80131044)

Tried using an older assembly dll (recaptcha-dotnet-1.0.2.0-binary.zip) as I though there could be compatibility issues of the recent reCaptcha library with the older .net frameworks using the following link,

https://code.google.com/archive/p/recaptcha/downloads

But I still get the same error,

Tried signing the assembly file in VS 2017 by going through "Properties" -> "Signing" and ticking the "Sign the Assembly" option. Now one thing I couldn't do was to create a "*.snk" file and link it to the Recaptcha.dll file. There is no mention of creating such file in Google docs but anyway I even tried the following instructions,

https://docs.microsoft.com/en-us/dotnet/framework/app-domains/how-to-create-a-public-private-key-pair

But I don't seem to have the "sn -k" command setup under my VS command prompt.

I really need to get this reCaptcha working for this site as we are pressed for time.