Message "Back up disc is not available" appears when I try to back up or turn time machine on

We need a bit more info..

Do you have a Time Capsule or Airport Extreme with external disk?

Does the disk show up ok in Airport Utility?

If external drive is it formatted HFS+.. TM cannot backup to a drive in any other format.

Pondini is the expert on all things that go wrong with TM.

See http://pondini.org/TM/FAQ.html

Look at the setup of TC or airdisk.. under gory details, here. http://pondini.org/TM/Home.html