This error is due to the account's disk space usage being over 5GB. You are able to adjust this limit for backups on demand by navigating to JetBackup >> Settings >> Backup on Demand and changing the "Automatically disable backup on demand in the following cases" setting for disk space and/or inode count.
To clarify further, JetBackup disables backups on demand if any one of the two conditions (exceeding disk space or inode count) is met.
For more information, please refer to our documentation: