Output requests can fail with message 'Frontend Unavailable' or 'Unable to connect to output processor' when an application tries to print from background to a Frontend printer, or prints from Update.
It is not possible to print via a frontend access method from a background job as there is no connection to the frontend when running in background mode. This is also mentioned in section 3,c(Restrictions) of the Note: 128105 - Frontend printing (collective note)
Frontend printing from a batch process cannot work, as there is no GUI connection. Make sure the user's steps when printing do not take place via BTC (batch) processes.