When running gencal to produce antenna offsets, the following warning and error may be generated:
2013-02-28 18:55:35 WARN gencal::::casa No offsets found for this MS
*** Warning *** No offsets found. No caltable created.
2013-02-28 18:55:35 SEVERE gencal::::casa An error occurred running task gencal.
This simply means that when gencal queried the online antenna offset database, it was not able to find any offsets for the MS it was given. Without any antenna offsets, it cannot create -- *nor is there any need for* -- an antenna position correction table in this case.