An Azure service that is used to control and help secure email, documents, and sensitive data that are shared outside the company.
It's recommended to use the built-in OME templates, as detailed here: https://learn.microsoft.com/en-us/microsoft-365/compliance/define-mail-flow-rules-to-encrypt-email?view=o365-worldwide
If for some reason you want to use a different template, make sure its published (make sure its visible in the output of Get-RMSTemplate). If it's not listed there, check your label policies. Only labels with "encrypt" action will be published to Exchange, and IIRC you need to scope it to all users.