how to erase the data's on the tape

"I need to delete that backups from the tape.Its showing some mounts on the volume. If I give the quick erase in the GUI, It's throwing error like"bplable on the tape cannot revoke remotely". can any one help me to resolve this? "

Once you write a backup to the tape, NB now keeps track of the fact that you have data on the tape.  This is the stuff you see in the BAR.  NB should not let you just erase a tape when it thinks you have valid backups on it.  (this is for your protection).
What you must do to make the tape a scratch is expire the images on the tape, this will cause NB to remove any reference to images on the tape from its database.
As stated above this can be done with the bpexpdate command

<installdir>/netbackup/bin/admincmd/bpexpdate -m <mediaid> -d 0

This will expire ALL images on the tape making it a scratch tape with no assigned date (it will NOT zero out the mounts on the tape, but you don't need to do that anyway)


and how can I view the cartridge size? and free size of a cartridges?

Also stated above you cannot see how much room is avail on a tape, as compression can very, and you really cannot know  how much room is left.
But you can get an idea by looking at the Tape Lists (in the gui under tape reports)
here run the report for all tapes
in the Status column look for any tape that says FULL and look at kilobytes, this will give you and idea of about how much you can get on a tape.
Now look at your tape in question and you can see about how much you have left -BUT THERE IS A BIG EXCEPTION TO THIS!
If you have many images on a tape and one of the images expires you cannot then write to the empty space of that images.
See my detail explanation on this at this link.

https://www-secure.symantec.com/connect/blogs/understanding-how-netbackup-writes-tape