Please try, to make your questions with a self-contained REPRoducible EXample (reprex)? A reprex makes it much easier for others to understand your issue and figure out how to help.
I think you are modeling the three Items as just one, you can reshape your data frame and model each one separately or nest your data and use purrrpackage to fit all the models.
df <- data.frame(stringsAsFactors = FALSE,
Item.Number = as.factor(c("004-0013", "DP-023-0059", "DP-023-0059",
"502-00038R", "502-00038R", "502-00038R",
"502-00038R", "502-00038R", "502-00038R", "502-00038R",
"004-0013", "004-0013", "502-00038R", "DP-023-0059",
"DP-023-0059", "DP-023-0059", "DP-023-0059",
"502-00038R", "502-00038R", "502-00038R", "502-00038R",
"502-00038R", "004-0013", "DP-023-0059", "DP-023-0059",
"DP-023-0059", "DP-023-0059", "DP-023-0059",
"DP-023-0059", "DP-023-0059", "DP-023-0059", "DP-023-0059",
"DP-023-0059", "502-00038R", "502-00038R",
"502-00038R", "502-00038R", "502-00038R", "502-00038R",
"502-00038R", "502-00038R", "502-00038R", "DP-023-0059",
"DP-023-0059", "DP-023-0059", "DP-023-0059",
"DP-023-0059", "DP-023-0059", "DP-023-0059", "DP-023-0059",
"DP-023-0059", "DP-023-0059", "DP-023-0059",
"DP-023-0059", "DP-023-0059", "DP-023-0059", "502-00038R",
"502-00038R", "502-00038R", "502-00038R",
"DP-023-0059", "DP-023-0059", "DP-023-0059", "502-00038R",
"502-00038R", "502-00038R", "502-00038R",
"DP-023-0059", "502-00038R", "502-00038R", "502-00038R",
"502-00038R", "DP-023-0059", "DP-023-0059", "502-00038R",
"502-00038R", "004-0013", "004-0013", "004-0013",
"004-0013", "004-0013", "004-0013", "004-0013",
"004-0013", "004-0013", "004-0013", "004-0013", "004-0013",
"004-0013", "004-0013", "004-0013", "004-0013",
"004-0013", "004-0013", "004-0013", "004-0013",
"004-0013", "004-0013", "004-0013", "004-0013", "004-0013",
"004-0013", "004-0013", "004-0013", "004-0013",
"004-0013", "004-0013", "004-0013", "004-0013")),
Month = c("2017-07-01", "2017-12-01", "2018-01-01", "2018-11-01",
"2019-01-01", "2018-06-01", "2018-08-01", "2018-05-01",
"2018-09-01", "2018-07-01", "2018-03-01", "2016-07-01",
"2018-12-01", "2018-12-01", "2017-04-01", "2018-11-01", "2016-05-01",
"2016-05-01", "2018-03-01", "2018-02-01", "2017-03-01",
"2016-03-01", "2016-08-01", "2017-06-01", "2016-11-01", "2017-07-01",
"2016-09-01", "2017-10-01", "2018-05-01", "2016-08-01",
"2016-06-01", "2018-03-01", "2016-02-01", "2017-04-01",
"2017-01-01", "2016-06-01", "2017-07-01", "2016-07-01", "2017-10-01",
"2018-04-01", "2017-05-01", "2016-04-01", "2016-03-01",
"2017-08-01", "2018-02-01", "2017-09-01", "2018-08-01", "2016-04-01",
"2018-06-01", "2016-01-01", "2018-07-01", "2018-04-01",
"2017-11-01", "2016-07-01", "2017-01-01", "2016-12-01", "2018-01-01",
"2016-02-01", "2016-09-01", "2016-08-01", "2017-02-01",
"2017-05-01", "2018-10-01", "2017-11-01", "2017-08-01",
"2016-12-01", "2017-02-01", "2018-09-01", "2016-01-01", "2017-06-01",
"2017-09-01", "2016-10-01", "2017-03-01", "2016-10-01",
"2017-12-01", "2016-11-01", "2018-11-01", "2018-12-01", "2017-02-01",
"2019-01-01", "2016-11-01", "2018-02-01", "2017-03-01",
"2017-05-01", "2017-08-01", "2018-07-01", "2016-09-01", "2016-02-01",
"2018-10-01", "2018-06-01", "2018-01-01", "2017-04-01",
"2017-10-01", "2016-10-01", "2018-05-01", "2017-01-01",
"2016-04-01", "2017-06-01", "2017-11-01", "2018-04-01", "2016-03-01",
"2016-12-01", "2016-06-01", "2016-01-01", "2017-12-01",
"2018-08-01", "2017-09-01", "2018-09-01", "2016-05-01"),
Qty = c(1L, 1L, 1L, 73L, 738L, 358L, 751L, 697L, 1400L, 210L, 4L,
4L, 1832L, 2L, 2L, 3L, 3L, 197L, 302L, 275L, 291L, 359L, 9L,
4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 365L, 297L, 590L,
380L, 418L, 438L, 288L, 369L, 237L, 6L, 5L, 5L, 5L, 5L, 5L, 7L,
6L, 6L, 6L, 6L, 6L, 6L, 6L, 1483L, 306L, 420L, 534L, 9L, 8L,
8L, 492L, 723L, 445L, 1544L, 9L, 619L, 679L, 829L, 517L, 8L, 8L,
313L, 867L, 14L, 15L, 14L, 19L, 30L, 13L, 9L, 4L, 15L, 13L,
16L, 17L, 6L, 7L, 6L, 11L, 17L, 21L, 13L, 12L, 24L, 11L, 12L,
14L, 13L, 12L, 16L, 6L, 9L, 12L, 21L, 6L, 12L)
)
library(tidyr)
library(dplyr)
df %>%
spread(Item.Number, Qty) %>%
mutate(Month = as.Date(Month)) %>%
arrange(Month) %>%
head()
#> Month 004-0013 502-00038R DP-023-0059
#> 1 2016-01-01 6 619 6
#> 2 2016-02-01 17 306 4
#> 3 2016-03-01 13 359 6
#> 4 2016-04-01 24 237 5
#> 5 2016-05-01 12 197 3
#> 6 2016-06-01 16 590 4
Created on 2019-03-08 by the reprex package (v0.2.1)