RMAN Recovery In Oracle 11g : Backup Database To Tape
The "device" in RMAN is a misnomer, it should be really called "storage".

The "sbt" (synonym of "sbt_tape") is a misnomer again, as it has NOTHING to do with any tapes, it should be simply called "non-rman". This is just an empty placeholder, to be filled with any "plugin"; the plugin is called by Oracle either the "Media Manager library" or SBT_LIBRARY. This plugin allows rman to store and retrieve files through it, so rman only tells that it needs a file handle "xyz" (file is identified by a string handle) and doesn't need to know how the file is delivered, from tape or anything. The plugin is normally a part of an independent backup software, such as IBM TSM or Symantec NetBackup or many others. Oracle provides a simple emulator SBT_LIBRARY=oracle.disksbt for testing.

Since you didn't fill that placeholder with any "Media Manager library", you receive an error message as expected.