Office 365 Mail Encryption
Your user is required to have "Azure Information Protection Plan Premium P1" or a license that includes this.
Powershell
Get-OMEConfiguration
Get-IRMConfiguration
Enable Encrypt Button
Set-IRMConfiguration -SimplifiedClientAccessEnabled $true
Enable AIPService
This may resolve RMS related errors?
Enable-AipService
$EndPoint=(Get-AipServiceConfiguration).LicensingExtranetDistributionPointUrl
Set-IRMConfiguration -LicensingLocation $EndPoint
Set-IRMConfiguration -InternalLicensingEnabled:$true -AzureRMSLicensingEnabled:$true -AutomaticServiceUpdateEnabled:$true -SimplifiedClientAccessEnabled:$true
Test it all
Test-IRMConfiguration -Sender user@acme.com -Recipient user@contos.com