get-childitem : Cannot find path ' ' because it does not exist - powershell

Question

I am trying to make a script that can report on a folder size for a specified folder.
I keep getting this error after I put in the folder name and I know the folder exists.
Also if I don't get this error then I always get 0 MB.
get-childitem : Cannot find path 'C:\WINDOWS\system32\downloads' because it does not exist.
At C:\Users\Erik\Desktop\powershell script.ps1:58 char:12
+ $folders = get-childitem $startfolder | where{$_.PSiscontainer -eq "T ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : ObjectNotFound: (C:\WINDOWS\system32\downloads:String) [Get-ChildItem], ItemNotFoundException
+ FullyQualifiedErrorId : PathNotFound, Microsoft.PowerShell.Commands.GetChildItemCommand
Code
$startfolder = Read-Host -Prompt 'Please enter the folder name:'
#check that input is not empty
if([string]::IsNullOrEmpty($startfolder)) {
Write-Host "folder name is NULL or EMPTY"
} else {
$folders = get-childitem $startfolder | where {$_.PSiscontainer -eq "True"}
"folder Name`tfolder Size (MB)"
foreach ($fol in $Folders) {
$colItems = (Get-ChildItem $fol.fullname -recurse | Measure-Object -property length -sum)
$size = "{0:N2}" -f ($colItems.sum / 1MB)
"$($fol.name)`t$size"
}
}

Answer 1

so.. My power-shell is a bit terrible but I've got this working for me:
$startfolder = Read-Host -Prompt 'Please enter the folder name:'
#check that input is not empty
if([string]::IsNullOrEmpty($startfolder)) {
Write-Host "folder name is NULL or EMPTY"
}
else {
$subFolderItems = Get-ChildItem $startfolder.FullName -recurse -force | Where-Object {$_.PSIsContainer -eq $false} | Measure-Object -property Length -sum | Select-Object Sum
$startfolder + " -- " + "{0:N2}" -f ($subFolderItems.sum / 1MB) + " MB"
}
the output is like:
test -- 2.65 MB