Edits to Script

$credentials = Get-Credential

#Defining import and export variables
$complist = ".\computer.txt"
$outreport = ".\Server_Inventory_" + $((Get-Date).ToString('MM-dd-yyyy')) + ".csv"

#Start PSRemoting.
$invokeArgs = @{
    ComputerName = (Get-Content $complist)
    Credential   = $credentials
}
[array]$allResults = Invoke-Command @invokeArgs -ScriptBlock {

    # NOTE -- I'm assuming you're using 'Get-WmiObject' for backwards
    #         capability reasons?  Instead of 'Get-CimInstance'?

    #Run the commands concurrently for each server in the list
    $CPUInfo = Get-WmiObject Win32_Processor #Get CPU Information
    $OSInfo = Get-WmiObject Win32_OperatingSystem #Get OS Information

    #Get Memory Information. The data will be shown in a table as GB, rounded to the nearest second decimal.
    $PhysicalMemory = Get-WmiObject CIM_PhysicalMemory | Measure-Object -Property capacity -Sum | ForEach-Object { [math]::round(($_.sum / 1GB), 2) }

    #Get Network Configuration
    $Network = Get-WmiObject Win32_NetworkAdapterConfiguration -Filter 'ipenabled = "true"'

    #Get local admins.
    # -- This 'will' fail on AD Domain Controllers, but can be ignored.
    $localadmins = Get-LocalGroupMember -Group "Administrators"

    #Get Powershell Version
    $powershell = $PSVersionTable

    #Get Filesystems.
    # -- '$Disk' doesn't seem to be used anywhere?
    $Disk = Get-WmiObject -Class Win32_logicaldisk -Filter "DeviceID = 'C:'" | Measure-Object -Property FreeSpace -Sum | ForEach-Object { [math]::round(($_.sum / 1GB), 2) }

    $drives = [System.Collections.Generic.List[object]]@(Get-WmiObject Win32_LogicalDisk -Filter "DriveType = 3")

    #Get list of shares
    $Shares = Get-WmiObject Win32_share |
        Where-Object { ($_.name -NotLike "*Admin*" -and $_.name -NotLike "*C*" -and $_.name -NotLike "*IPC*") }

    return [pscustomobject]@{
        ServerName           = $CPUInfo.SystemName
        "CPU_Core_Count"     = $CPUInfo.NumberOfCores
        "TotalMemory_GB"     = $PhysicalMemory
        "OS_Name"            = $OSInfo.Caption
        "OS_Version"         = $OSInfo.Version
        "IP Address"         = $Network.IPAddress
        "Local Admins"       = $localadmins.Name
        "PowerShell Version" = $powershell.PSVersion.ToString()
        SharesName           = $Shares.Name
        SharesPath           = $Shares.Path
        Drives               = $drives
    }
    <#
    Your original code
    $infoObject = New-Object PSObject

    #Add data to the infoObjects.
    Add-Member -InputObject $infoObject -MemberType NoteProperty -Name "ServerName" -Value $CPUInfo.SystemName
    Add-Member -InputObject $infoObject -MemberType NoteProperty -Name "CPU_Core_Count" -Value $CPUInfo.NumberOfCores
    Add-Member -InputObject $infoObject -MemberType NoteProperty -Name "TotalMemory_GB" -Value $PhysicalMemory
    Add-Member -InputObject $infoObject -MemberType NoteProperty -Name "OS_Name" -Value $OSInfo.Caption
    Add-Member -InputObject $infoObject -MemberType NoteProperty -Name "OS_Version" -Value $OSInfo.Version
    Add-Member -InputObject $infoObject -MemberType NoteProperty -Name "IP Address" -Value $Network.IPAddress
    Add-Member -InputObject $infoObject -MemberType NoteProperty -Name "Local Admins" -Value $localadmins.Name
    Add-Member -InputObject $infoObject -MemberType NoteProperty -Name "Powershell Version" -Value $powershell.PSversion
    Add-Member -InputObject $infoObject -MemberType NoteProperty -Name "SharesName" -Value $Shares.Name
    Add-Member -InputObject $infoObject -MemberType NoteProperty -Name "SharesPath" -Value $Shares.Path

    # foreach ($drive in $drives)
    # {
    #     if ($drive.size -gt 0)
    #     {
    #         $drivename = $drive.DeviceID
    #         $freespace = [int]($drive.FreeSpace / 1GB)
    #         $totalspace = [int]($drive.size / 1GB)
    #         Add-Member -InputObject $infoObject -MemberType NoteProperty `
    #       -Name "$drivename Free Space" -Value ("$freespace GB / $totalspace GB")
    #     }
    # }

    $infoObject
    #>
}

# Get all unique Drive ID's
# -- This will provide the maximum list of 'FreeSpace' columns to add.
[string[]] $uniqueIds = $allResults.Drives.DeviceID | Select-Object -Unique

foreach ($result in $allResults) {

    # Store the computer's drives' DeviceID's.
    [string[]] $deviceIDs = $result.Drives.DeviceID

    # Find which Drive ID's are present and not present on the
    # current computer result.
    $idsPresent, $idsNotPresent = $uniqueIds.Where( { $_ -in $deviceIDs }, "Split")

    # Create a dummy PSNoteProperty for the drives that are
    # 'NOT' present on the computer.
    foreach ($notPresent in $idsNotPresent) {
        $propName = "$notPresent FreeSpace"
        [void] $result.psobject.Properties.Add(
            (New-Object PSNoteProperty($propName, $null))
        )
    }

    # Create a real PSNoteProperty for the drives that
    # 'ARE' present on the computer.
    foreach ($present in $result.Drives.Where( { $_.DeviceID -in $idsPresent })) {
        $propName = "$($present.DeviceID) FreeSpace"
        $freespace = [int]($present.FreeSpace / 1GB)
        $totalspace = [int]($present.Size / 1GB)
        $propValue = "$freespace GB / $totalspace GB"

        [void] $result.psobject.Properties.Add(
            (New-Object PSNoteProperty($propName, $propValue))
        )
    }
}

# Splat the Select-Object parameters.
$selectArgs = @{
    Property        = '*'
    ExcludeProperty = @(
        'Drives'
        'PSComputerName',
        'RunspaceId',
        'PSShowComputerName'
    )
}

# Export to CSV.
$allResults | Select-Object @selectArgs | Export-Csv -Path $outreport -NoTypeInformation

# Original Code
#Select-Object * -ExcludeProperty PSComputerName, RunspaceId, PSShowComputerName | Export-Csv -Path $outreport -NoTypeInformation