Moving forecast and Holtwinters function

Hello to all
MOVING FORECAST
I am trying to get a matrix with a moving forecast bases on previous 20 observations. (sales of water packs.)

HOLT WINTERS VALUE OR ETS
In my code i also have a calculation which shows me what the Holt winters numbers are for the whole time serie.(2 observation)But i would need to see those showing on a line bases (moving value per rolling 20 observations).

Is there anyone who can tell me what code ineed?

reprex

# load needed packages
library(readr)
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
library(readxl)
library(forecast) # necessary to use forecasting methods
#> Registered S3 method overwritten by 'quantmod':
#>   method            from
#>   as.zoo.data.frame zoo
library(tsintermittent) # TSB (Teunter-Syntetos-Babai) method
#> Loading required package: MAPA
#> Loading required package: parallel
#> Loading required package: RColorBrewer
#> Loading required package: smooth
#> Loading required package: greybox
#> Error: package or namespace load failed for 'greybox' in loadNamespace(j <- i[[1L]], c(lib.loc, .libPaths()), versionCheck = vI[[j]]):
#>  there is no package called 'RcppParallel'
#> Error: package 'greybox' could not be loaded
library (ggplot2)

# import data

sales <-
  read_excel("C:/Users/jenny/Desktop/R Studio/MonthlySales1.xls")

# explore data
str(sales)
#> Classes 'tbl_df', 'tbl' and 'data.frame':    95 obs. of  8 variables:
#>  $ Month   : chr  "january" "february" "march" "april" ...
#>  $ sales   : num  43 48 47 48 48 49 38 25 48 48 ...
#>  $ Forecast: num  NA NA NA NA NA NA NA NA NA NA ...
#>  $ fc alpha: num  NA NA NA NA NA NA NA NA NA NA ...
#>  $ fc beta : num  NA NA NA NA NA NA NA NA NA NA ...
#>  $ fc gamma: num  NA NA NA NA NA NA NA NA NA NA ...
#>  $ Low     : num  NA NA NA NA NA NA NA NA NA NA ...
#>  $ High    : num  NA NA NA NA NA NA NA NA NA NA ...

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