I've been following these instructions to extend the Health Check dashboard: https://our.umbraco.org/documentation/Extending/Healthcheck/

However, the "Fix" button is disabled and I can't figure out how to get it to be enabled:

Some relevant info:

• There are no errors in the JavaScript console.
• Umbraco 7.5.13.

Here's the code I'm using:

using System.Collections.Generic;
using Umbraco.Web.HealthCheck;
using Umbraco.Web.HealthCheck.Checks.Config;

[HealthCheck("B878A830-6DE4-4352-9283-4B867DCF7C75",
    "Check Robotnik", Description = CheckDescription, Group = "Live Environment")]
public class RobotnikCheck : AbstractConfigCheck
{

    private const string CheckDescription = "Checks the Robotnik configuration in the web.config to ensure you aren't still using the default sample domain.";

    public RobotnikCheck(HealthCheckContext healthCheckContext) : base(healthCheckContext)
    {
    }

    public override string FilePath => "~/Web.config";

    public override ValueComparisonType ValueComparisonType => ValueComparisonType.ShouldNotEqual;

    public override IEnumerable<AcceptableConfiguration> Values => new[]
    {
        new AcceptableConfiguration()
        {
            IsRecommended = false,
            Value = "robotnik-www.some-site.com"
        }
    };

    public override string XPath => @"/configuration/appSettings/add[@value=""~/robots-primary.txt""][1]/@key";

    public override string CheckSuccessMessage => "The sample Robotnik domain is no longer being used. That probably means you've replaced it with the correct domain.";

    public override string CheckErrorMessage => @"The sample Robotnik domain is still being used. You should change it to the correct domain. For example, if you are working with www.microsoft.com, the value would be ""robotnik-www.microsoft.com"".";

    public override string RectifySuccessMessage => "The Robotnik domain has been updated to the one you specified. Edit the web.config if you have more domains to add.";

}

How do I fix the "Fix" button?