The Last Job writing to that tape probably failed with "cannot write to device" or "cannot write filemark".
Therefore the last Data-Segment is not finalized with a Trailing Filemark.
The Tapedrive tries to find that last Filemark to append new data but cannot find it. To avoid corrupting any data the write attempt is denied an the Media is set to poor.
If the Filemark is there but just not on the position registered in IDB a Verify can help recover it.
If it is missing you may want to copy away all vaild Objects on the Media and reformat it. Failed Objects and the last Object written during the Fault will be lost after that.