How to write a script to copy files and folders with sub-folders last written or updated 20 days before into 7zip file

Get-ChildItem F:\log | where-object {$i.lastwritetime -le ((Get-Date).AddDays(-1))} | Copy-Item -destination F:\new
$SFOLDER= "F:\log"
$DSTFOLDER= "f:\new\"
if (-not (test-path "$env:ProgramFiles\7-Zip\7z.exe")) {throw "$env:ProgramFiles\7-Zip\7z.exe needed"}
set-alias sz "$env:ProgramFiles\7-Zip\7z.exe"

Please find the script in which the files zipped are not as per the folder and file structure. Also i believe its not a perfect script.

Script error while running in prompt