RESOLVED: You can't create a rule containing the ApplyOME or RemoveOME action because IRM licensing is disabled.

Recently enabled Office Message Encryption (OME) through a transport rule for a tenant and found the following error:

"You can't create a rule containing the ApplyOME or RemoveOME action because IRM licensing is disabled."

After some research I discovered you need to enable the Information Rights Management (IRM) licensing as follows:

$LiveCred = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/  -Credential $LiveCred -Authentication Basic -AllowRedirection
Import-PSSession $Session

Set-IRMConfiguration -RMSOnlineKeySharingLocation https://sp-rms.na.aadrm.com/TenantManagement/ServicePartner.svc

Import-RMSTrustedPublishingDomain -RMSOnline -name "RMS Online"
Set-IRMConfiguration -InternalLicensingEnabled $True

Test-IRMConfiguration -RMSOnline

Once this is complete, you will need to wait a few hours for it to be enabled and your transport rule should complete just fine.

Message encryption reference: https://blogs.office.com/2013/11/21/introducing-office-365-message-encryption-send-encrypted-emails-to-anyone/