Hi,
condor_submit will automatically insert in the job classAD x509userproxy = VALUE_OF_ENV_VAR_X509_USER_PROXY for several cases such as universe = grid grid_type = gt4 grid_resource = gt4 $$(gatekeeper_url) $$(job_manager_type) but not for universe = grid grid_type = gt4 grid_resource = $$(resource_name) so it looks like I need to write a wrapper around condor_submit which will do the insertion.
you're absolutely right here, Condor should add this attribute (either by using default path or env variable).
Since I install development versions as they're released, at certain point (around 6.7.15 or 16) I got the same error as you. From that point I simply assumed that this attribute is mandatory and started adding it to all job scripts. Never bothered to dig a little deeper. Let's hope that next release will solve this problem...
Cheers, emir