I have repeatedly receiving tape error as follows
Which version of NBU you are using?
NBU will issue cleaning job to the drive with cleaning flag set on it, if NBU is not doing so this error may occur till you clean the drive manually.
Have you tried cleaning the drive manually from library interface?
Even if the error persists after cleaning the drive, you have to check the hardware functionality.
Give a read here: https://www-secure.symantec.com/connect/blogs/facing-any-issues-your-tape-device-try
See if any cleaning jobs issued to tape drive from NBU admin console. If none, try cleaning the tape drive manually from library interface. If a cleaning job is already issued by NBU, then the problem might be with the hardware. If the drive is of IBM make you can use IBM ITDT tool to test it. (Can be downloaded from IBM site).
You need cleaning tapes in the robot - either in the robot's maintenance slots (if that is available - see your robot's documentation) or in 'normal' slots that NetBackup knows about.
If we go the 'normal' slot route - ensure that your cleaning tapes have unique labels - normally starting with CLN. Add these cleaning tapes manually using steps in this TN http://www.symantec.com/docs/TECH19399 or add a Barcode Rule for cleaning tapes - see http://www.symantec.com/docs/TECH37409.
See http://www.symantec.com/docs/HOWTO56118 for explanation of cleaning TapeAlert messages.