Custom tag helper not working

See Managing Tag Helper scope

@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
@addTagHelper *, AuthoringTagHelpers

The code above uses the wildcard syntax (*) to specify that all Tag Helpers in the specified assembly (Microsoft.AspNetCore.Mvc.TagHelpers) will be available to every view file in the Views directory or sub-directory.

You need to provide only assembly name in the view imports file.

_ViewImports.cshtml:

@addTagHelper *, ToolConstrolSystem