$e1 = '<PATH TO previous_month.xlsx>'
$e2 = '<PATH TO current_month.xlsx>'

# Copy the worksheet from previous_month.xlsx to current_month.xlsx
$open_e2 = Import-Excel $e1 -worksheet 'old' | export-excel -Append $e2 -WorksheetName 'old' -PassThru

$rows = $open_e2.new.Dimension.Rows

<#
for ($i=2; $i -le $rows; $i++) {
  # $open_e2.new.Cells["D$i"].Formula = "=INDEX(old!D:D,MATCH(1,(old!C:C=[@[Plugin_Name]])*(old!B:B=[@Type])*(old!A:A=[@[Year_Month]]),0))"
  # OR
  # Set-ExcelRange -Worksheet $open_e2.new -Range "D$i" -Formula "=INDEX(old!D:D,MATCH(1,(old!C:C=[@[Plugin_Name]])*(old!B:B=[@Type])*(old!A:A=[@[Year_Month]]),0))"
}
#>

Set-ExcelRange -Worksheet $open_e2.new -Range "D2:D$($rows)" -Formula "=INDEX(old!D:D,MATCH(1,(old!C:C=[@[Plugin_Name]])*(old!B:B=[@Type])*(old!A:A=[@[Year_Month]]),0))"

Close-ExcelPackage $open_e2 -Calculate