I had the same issue building 4.1. The only way I was able to successfully build was omitting the template VMs and building only dom0, as mentioned here Error when running qubes-builder : Unable to detect release version use '--releasever' - #7 by xmpriv
However the ISO that gets built doesn’t boot, ELF error as described here ISO build for Qubes 4.1 is broken (ELF: Not an ELF binary)