This error occurs when someone has run a process that has broken the log chain. Probably ran the command BACKUP LOG [Jul_Reports] WITH TRUNCATE_ONLY, or the database was switched to SIMPLE recovery and then back to FULL.
Once the log chain is broken, you cannot perform transaction log backups until the log chain has been re-established. To do that, you need to perform either a full or diff backup on that database.
If you don't need the database in full recovery, switch to simple and stop performing transaction log backups.
SQL Database Recovery Expert :-)