C# – System.Collections.Generic.IEnumerable’ does not contain any definition for ‘ToList’

Here is the problem. I am getting IEnumerable from ViewPage and when I tried it to convert List it is showing me error like:

'System.Collections.Generic.IEnumerable<Pax_Detail>' does not contain
a definition for 'ToList' and no extension method 'ToList' accepting a
first argument of type
'System.Collections.Generic.IEnumerable<Pax_Detail>' could be found
(are you missing a using directive or an assembly reference?)

Here is my controller code:

[HttpPost]
public ActionResult Edit_Booking(Booking model, IEnumerable<Pax_Detail> pax)
{
  List<Pax_Detail> paxList = new List<Pax_Detail>();
  paxList = pax.ToList(); //getting error here
  BookingDL.Update_Booking(model, paxList);
  return View();
}

I have applied same logic on another controller. And it is working fine. I don't know what problem it has. I have already clean, rebuild project and also restarted my laptop(though it was needed).