Not sure when _amd64 was introduced. But should already exist. Dropping _amd64 works?
Any folders in /var/cache/pbuilder/ exist?
ls -la /var/cache/pbuilder/
cowbuilder debugging presupposes that build-steps.d/1100_prepare-build-machine was already run and successful.
build-steps.d/1100_prepare-build-machine was recently split, though.
https://github.com/Whonix/Whonix/commit/8cd7bfa8606755539abe46180530fd32c0a0c7ae
In other words as per git tag 15.0.0.4.3-developers-only the first three build steps
- build-steps.d/1100_sanity-tests
- build-steps.d/1120_prepare-build-machine
- build-steps.d/1130_cowbuilder-setup
should have already succeeded.