Script to enable bitlocker in All Drive

I am trying to enable bitlocker in all domain joined user machines in my office.

I have used a Widows task scheduler script to enable bitlocker in all machines.

But the below code is enabling bitlocker in C drive alone.

I need to enable this in all drive in the laptop. How do i proceed. I have attached the script below

$TPM = Get-WmiObject win32_tpm -Namespace rootcimv2securitymicrosofttpm | where {$_.IsEnabled().Isenabled -eq 'True'} -ErrorAction SilentlyContinue
$WindowsVer = Get-WmiObject -Query 'select * from Win32_OperatingSystem where (Version like "6.2%" or Version like "6.3%" or Version like "10.0%") and ProductType = "1"' -ErrorAction SilentlyContinue
$BitLockerReadyDrive = Get-BitLockerVolume -MountPoint $env:SystemDrive -ErrorAction SilentlyContinue


#If all of the above prequisites are met, then create the key protectors, then enable BitLocker and backup the Recovery key to AD.
if ($WindowsVer -and $TPM -and $BitLockerReadyDrive) {

#Creating the recovery key
Start-Process 'manage-bde.exe' -ArgumentList " -protectors -add $env:SystemDrive -recoverypassword" -Verb runas -Wait

#Adding TPM key
Start-Process 'manage-bde.exe' -ArgumentList " -protectors -add $env:SystemDrive  -tpm" -Verb runas -Wait
sleep -Seconds 15 #This is to give sufficient time for the protectors to fully take effect.

#Enabling Encryption
Start-Process 'manage-bde.exe' -ArgumentList " -on $env:SystemDrive -em aes256" -Verb runas -Wait

#Getting Recovery Key GUID
$RecoveryKeyGUID = (Get-BitLockerVolume -MountPoint $env:SystemDrive).keyprotector | where {$_.Keyprotectortype -eq 'RecoveryPassword'} | Select-Object -ExpandProperty KeyProtectorID

#Backing up the Recovery to AD.
manage-bde.exe  -protectors $env:SystemDrive -adbackup -id $RecoveryKeyGUID

#Restarting the computer, to begin the encryption process
Restart-Computer}

$drives = Get-Volume | Where {$_.DriveType -like "Fixed" -and $_.DriveLetter -ne $null -and $_.FileSystem -ne $null}
Foreach ($drive in $drives){
    $drive.DriveLetter
}