You need to pursue diagnosis.
Your posting doesn't say if this is the first time this client backup
has been historically attempted, or if it had been working for some
time.  It may be the first attempt, and the server configuration is
incorrect such that type of drive requested is one which does not
exist in the library.  Gather information from the TSM server
Activity Log regarding the initial session and exactly what it's
looking for in terms of drive and media type, and pursue that avenue
of investigation.  If you find server configuration correct, look
into the viability of paths and drives.

   Richard Sims