Dell has changed the name of its acquired AppAssure to Dell Data Protection Rapid Recovery. AppAssure (latest version is 5.4.3) is no longer being maintained, so upgrading to Rapid Recovery (latest version 6.0.2 at the time of writing) is a requirement for continued support. Fortunately, performing the upgrading is not really difficult, and can be done without too much downtime for your backup.

Additional information, including a compatibility and upgrade matrix can be found here:

How to Upgrade AppAssure 5.4 to Rapid Recovery 6.X (181997)

Dell Support Policy (compatibility matrix for Core and Agent versions)

At the time of writing, the compatibility matrix shows that Agent version 5.4.3  ...