Are you getting the error like this in System Center Date Protection Manager 2012:

The selected tapes cannot be marked as free because they belong to protection groups.
You need to stop protection of the associated protection groups before you can mark the tapes as free. The protection group that a tape belongs to is listed in the Protection Group column. (ID: 31116)

You can still force DPM to mark the tape as free by removing the recovery points. Use the following script in the DPM Powershell. In this case you are forcing and deleting all recovery points,

1. Open notepad, and copy/paste the give lines,

2. Do the changes as per your server name ($dpmserver) and tape barcode ($barcode)

3. Open DPM PowerShell and run the entries, it will do the job. Now you can run the inventory and do the tape as free.

$barcode = “000xxxx”
$dpmserver = “DPMSERVER”
$DPMLib = Get-DPMLibrary -DPMServerName $dpmserver
$tape = Get-Tape -DPMLibrary $DPMLib | Where-Object {$_.Barcode.Value -eq $barcode}
foreach ($RecoveryPoint in @(Get-RecoveryPoint -Tape $tape))
    Remove-RecoveryPoint -RecoveryPoint $RecoveryPoint -ForceDeletion -Confirm:$false

Note:  #type the barcode of the tape that shall be marked as free other wise you can leave as blank,

$barcode = ” ”