Could you tell me how the backup is taken now for the J2EE stack ?
If there is no backup scheduled then you need to configure the system with a backup server and also configure the .utl and .sap profiles for that.
Creating an entry in the crontab will not just help in taking the backup.
Unless and until the server is configured with a backup server no matter how the backups are started they will fail.
You need to first plug the system into a backup server with the help of a backup specialist.
You just need to put the backup script in the crontab using the command:
crontab -e
Before that I suggest to read these SAP notes.
They provide another option to configure backups using DB13 from an ABAP system.
1025707 - DBA Cockpit: Planning calendar and remote Oracle databases
853610 - Configuring transaction DB13C for Oracle
Regards
RB