This appliance was initially installed with a software release prior to 2.7.0, and has not had its datastore reset after 2.7.0 or later was installed.
The 3.0 software release only supports the new storage format introduced with 2.7.0, and cannot be installed without first performing a data reset (which will delete all content and recreate the datastore in the new format).
This can be done at any time before the appliance 3.0 release is installed.
A data reset will be required before the appliance 3.0 release can be installed. Be sure the backup system has been running for 48 hours without any failure reports before performing this reset, and that you have downloaded your backup encryption key.