This error normally means that there is another bzr process running that still holds the lock for the repository (possibly put in the background with Ctrl-Z).
You should find that process and resume or kill it. This should release the lock.
If you are absolutely, 100% positive that there is no such process (this shouldn't really happen on a local repository, because locks are automatically released when the bzr process terminates), you can try bzr break-lock
to break the lock regardless.
Note that breaking a lock should only be done as a last resort; if you forcibly break a lock and another process did still have access, then two bzr instances may be accessing the repository concurrently and corrupt the repository files. Breaking a lock should generally only be necessary when accessing a remote repository via a "dumb" protocol (sftp and friends). If you break a lock, it can be a good idea to back up the branch/repository first.
That the error message is not humanly readable is a known bug, I believe.