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The method .unit_group() is indeed missing for non-prime finite fields.
Implementing it is the object of
and that ticket even has a patch with an implementation,
which one can download and apply to one's installation of Sage.
The ticket has been stalled, but hopefully can be revived.
 Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.
 
                
                Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.