I warned about this problem here:
https://dietpi.com/forum/t/tinker-board-dietpi-release-is-missing-alsa-loopback/1704/1
The alsa loopback is not included in the OS (at least on my Tinker Board version), so BruteFIR can’t find it and throws the error.
Please join me in asking Fourdee to add the alsa loopback to all current DietPi releases as soon as possible.