The special Makefile targets 'tag', 'rpm', and 'bkradd' (and the other more obscure ones) are not documented anywhere.
On Gerrit: http://gerrit.beaker-project.org/2108
This is now included in the development version of Beaker's documentation: http://beaker-project.org/docs-develop/user-guide/task-makefile-targets.html
Verify this bug since no error is found.
Beaker 0.14.1 has been released.