If you use the tags-parameter in the potential, the potential acts only on the tagged atoms, not on the gold atoms, which do not belong to this tagged group. Without having seen your script, I suppose in your case not all atoms belong to the tagged group 'layer1' and that means that there is no potential defined for the rest of the system. This is what the error message means.
To fix it define another Tersoff potential for the remaining part of the system (assuming this part has the tag 'layer2'):
eam_layer1 = Tersoff_Au_2012(tags='layer1')
eam_layer2= Tersoff_Au_2012(tags='layer2')
calculator = TremoloXCalculator(parameters=[eam_layer1, eam_layer2])
If this does not solve your error, please post the script that you want to run.
Support for EAM-potentials and tags is something that we plan to add hopefully soon.