Fdbbackup with a new tag and the result is restorable: false

I run the fdbbackup with -d file option and with a new tag using the -t option. The database has TLS enabled and I include the TLS key file and the backup was run successfully. However, when I use fdbbackup describe on the backup file, it said it is not restorable. There is only one snapshot as I use a new tag and the others I took the default. So, what could be wrong and how to make the backup restorable?

More update.   It seems there is some issue with the backup.  We have about 50GB data file but the backup set the backup created only amount to 15GB.  Then the next try it stopped at 5GB.  We don't see any error message related to IO or anything like that.  The only thing we see is the N2_ReadProbeError but would that be the issue which aborted the backup?