The tape leader is a band of flexible plastic with a hook on the end that is attached to the reel inside the drive.
When a damaged tape is used in the drive or when the drive is faulty, the tape leader can miss hook the leader and it pulls through and just flaps around on the internal spool leaving the drive unable to accept any tapes and failing self test.
Re-thread the lape leader round the guides and past the head and hook the square hole in the leader back onto the plastic hook just in front of where the tape usually sits and power on the drive and see if it passes self test.
Although this is often just a damaged tape that causes this it can also suggest a faulty drive so dont use any important tapes in the unit till you are sure the drive isnt totally 100%
I have used www.tape-drive-repair.com
who are pretty good for this kind of thing and not too expensive - if you need to send the drive away for repair / exchange