also, are you running it on Windows? if you do, you might need to make sure you are running it as Admin.
i seem to also remember somebody having this problem when downloading the flash command directly from git (i think the OS is trying to prevent running something that was downloaded), which can be solved by downloading the zip file that is linked to the release, extracting it somewhere and then copying the flash command from there. alternatively, you can just create the flash command file yourself using a text editor, and just copy/paste this:
#!/bin/sh
cd "$(dirname "$0")"
dfu-programmer at32uc3b0256 erase
dfu-programmer at32uc3b0256 flash orca.hex --suppress-bootloader-mem
dfu-programmer at32uc3b0256 start