您需要强制转换为实现IActionResult驱动程序类。请尝试此操作。

public IActionResult DeluxRoomCount()
{
   string deluxRoom = "select COUNT(type) from Rooms where Type='Delxu' And Avilability='True'";
   int count = 0;
   using (SqlConnection thisConnection = new SqlConnection("Data Source=HootelReservationDb1"))
        {
           using (SqlCommand cmdCount = new SqlCommand(deluxRoom, thisConnection))
                {
                    thisConnection.Open();
                    count = (int)cmdCount.ExecuteScalar();
                }
        }

    return Ok(count)
 }

你的方法的返回类型是IActionResult，但是你返回的是一个int，为了让你的方法正常工作，你需要返回一个IActionResult。下面是一些关于如何做的参考：https://learn.microsoft.com/en-us/aspnet/core/web-api/action-return-types?view=aspnetcore-6.0