There are different types of data at rest:

• Configuration data (at the moment mostly stored in files on disk)
• DB Data (MySQL, Cassandra, Elastic)

At the moment there is no feature to enable encryption through DataMiner for data at rest.
You could configure the OS to encrypt everything on disk, which would encrypt all data on disk.
Keep in mind that this will reduce the performance especially for your DB as that one will use the disk very intensively.
Another thing is that if an attacker has access to the system through a user account encrypting the disk through your OS will not give much added security.