Does it mean that each BKF file is 30GB, if the backup data reach 30GB then it creates another BKF file
Yes, once the bkf file reaches the maximum size specified, BE will create another bkf file to continue backups. Also if you select "Allocate maximum size for backup-to-disk files" option, BE creates a bkf file of size 30 GB irrespective of the size of data written to it. I prefer not to select this option.
Your backup job may be failed for some other reason, not because of bkf file size. Check the job log for any other errors.
Regards....