: Modern N64 emulators like Project64 or Mupen64Plus are designed to read .z64 files directly. They do not recognize .iso files as valid game data.
ucon64 -bin input.z64 output.bin
Do not convert. Use native Z64 format with any modern N64 emulator. z64 to iso
ucon64 -i input.z64 -o output.n64