The correct overload is (string linkText, string actionName, string controllerName, object routeValues, object htmlAttributes)

Your incorrect overload points to (string linkText, string actionName, object routeValues, object htmlAttributes)

The overload you want to use requires 5 arguments, but you only pass in 4.  It should be:

@Html.ActionLink("Check Availability", "CheckAvail", "Booking", new { id = item.ShowId }, null)