Saw the same thing and the answer is:
Symptom: Under certain conditions a ClearPass backup fails and the error message “ERROR - Failed to backup extensions: ERROR: Backup extensions: Extensions service is disabled, extensions will not be backed up” is displayed.
Scenario: If you do not use Extensions functionality, this issue will not affect your backup and the error message can be ignored. This issue only occurs if the Extensions service is not running during a backup or make-subscriber operation. In this case, any installed Extensions will not be included, but the rest of the backup will proceed normally. The Extensions service must be running during a backup or make-subscriber operation in order to include Extensions in the backup file.
Workaround: If you have ClearPass Extensions installed and you need to back them up — for example, if you are upgrading to the next major version or if you are migrating to a different 6.6.8 server — ensure that the Extensions service is running during a backup or make-subscriber operation.