ASP.NET Core 2.1 SignalR is not defined

I see your application is ASP.NET Core but you are using jquery.signalR.js which is actually for ASP.NET MVC. SignalR client for ASP.NET Core is signalr.js which is independent of jQuery. You can download the signalr.js for ASP.NET Core from NPM:

You can also download using Visual Studio Client Side Library Manager (LibMan) as follows:

• In Solution Explorer, right-click on the project, and select Add > Client-Side Library.

• In the Add Client-Side Library dialog, for Provider select unpkg.

• For Library, enter @microsoft/signalr@latest, and select the latest version
  that isn't preview.

• Must change Target Location to wwwroot/lib/signalr if it is anything else containing @ in the path. Otherwise it would not download.

For more details: Getting started with ASP.NET Core SignalR

You're using the wrong signalr javascript library. Try https://www.npmjs.com/package/@aspnet/signalr, that's what worked for me.