I don't know how to change it. Arima Model prediction of R language

There's still a lot of data out there, and the program hasn't been completely done yet.

> read.xlsx ("h:/projectpaper/paper/1.xlsx", "Sheet1")

> item<-read.xlsx ("h:/projectpaper/paper/1.xlsx", "Sheet1")

> item<-TS (item,start=c (2014))

> Plot.ts (item)

> itemdiff<-diff (item,differences=1)
> plot.ts (Itemdiff)
> itemdiff2<-diff (item,differences=2)
> plot.ts (ITEMDIFF2)
> itemdiff3<-diff (item,differences=3)
> plot.ts (ITEMDIFF3)
> ACF (ITEMDIFF2,LAG.MAX=20)
> ACF (itemdiff2,lag.max=20,plot=false)

Autocorrelations of series ' itemdiff2 ', by lag

0 1 2 3 4 5 6 7
1.000-0.668 0.177 0.004-0.111 0.255-0.289 0.275
8 9 10 11 12 13 14
-0.282 0.204-0.016-0.191 0.281-0.197 0.058

> pacf (itemdiff2,lag.max=20)
> pacf (itemdiff2,lag.max=20,plot=false)

Partial autocorrelations of series ' itemdiff2 ', by lag

1 2 3 4 5 6 7 8 9
-0.668-0.484-0.339-0.499-0.208-0.297 0.061-0.068 0.011
10 11 12) 13 14
0.140-0.142-0.169-0.033-0.148

> Itemarima<-arima (Item,order=c (1,2,1))
> Itemarima

Call:
Arima (x = Item, order = C (1, 2, 1))

Coefficients:
AR1 MA1
-0.4631-1.0000
S.E. 0.2145 0.2463

Sigma^2 estimated as 602016:log likelihood = -122.96, AIC = 251.92

> Library (forecast)
> Itemarimaforecast<-forecast (itemarima,h=5,level=c (99.5))
> Itemarimaforecast

Point Forecast Lo 99.5 Hi 99.5
2031 5777.286 3529.572 8025.001
2032 5780.032 3164.663 8395.400
2033 5818.222 2592.498 9043.945
2034 5839.998 2169.228 9510.768
2035 5869.375 1721.669 10017.081
> Plot.forecast (itemarimaforecast$residuals)
Error in Plot.forecast (itemarimaforecast$residuals):
There's no "plot.forecast" function.
> ACF (ITEMARIMAFORECAST$RESIDUALS,LAG.MAX=20)
> Box.test (itemarimaforecast$residuals, lag=20, type= "Ljung-box")

Box-ljung Test

Data:itemarimaforecast$residuals
x-squared = na, df = a, P-value = NA

> plot.ts (itemarimaforecast$residuals)

I don't know how to change it. Arima Model prediction of R language