HOW to find the avg difference in order time in SQL

Question

the dataset of subquery is here

id   itm_id   paid_at          ord_r total_r
17  3266    2013-05-25 08:27:17 1   3
17  3219    2013-05-25 08:27:17 2   3
17  3964    2013-05-25 08:27:17 3   3
25  2105    2013-05-17 03:11:48 1   2
25  1376    2013-05-17 03:11:48 2   2
63  2140    2013-07-07 11:26:45 1   3

the code is here

for find out the average difference in order time, BUt i looked up here, and i found this piece of code But i didn't understand why i use (toatl-1) if someone kindly explain the process i am doing this on mode analytics i don't know which machine it use, whether mysql or sqlserver

SELECT 
  user_id,
  item_id,
  CASE WHEN total_order-1 > 0
  THEN datediff(day, max(paid_at), min(paid_at))/ (total_order-1)
  ELSE datediff(day, max(paid_at), min(paid_at)) END AS avg_time
  FROM

(SELECT  
  user_id,
  item_id,
  paid_at,
  ROW_NUMBER( ) OVER (PARTITION BY user_id ORDER by paid_at ASC) AS order_rank,
  COUNT(item_id) OVER(PARTITION BY user_id ORDER BY paid_at ASC) AS total_order
from 
  dsv1069.orders) user_level

But the problems is ERROR: column "day" does not exist