Hello!
I've got the same issue despite following the hookup guide step by step.
I thought i would provide the full error message i get so maybe someone who understands it might help.
Arduino: 1.8.19 (Linux), Board: "SparkFun MicroMod nRF52840 Processor Board"
arduino-builder -dump-prefs -logger=machine -hardware /usr/share/arduino/hardware -hardware /home/mario/.arduino15/packages -tools /usr/share/arduino/hardware/tools/avr -tools /home/mario/.arduino15/packages -libraries /home/mario/Arduino/libraries -fqbn=arduino:mbed_nano:sfnrf52840pb -vid-pid=2341_805A -ide-version=10819 -build-path /tmp/arduino_build_879307 -warnings=default -build-cache /tmp/arduino_cache_16636 -prefs=build.warn_data_percentage=75 -prefs=runtime.tools.bossac.path=/home/mario/.arduino15/packages/arduino/tools/bossac/1.9.1-arduino2 -prefs=runtime.tools.bossac-1.9.1-arduino2.path=/home/mario/.arduino15/packages/arduino/tools/bossac/1.9.1-arduino2 -prefs=runtime.tools.openocd.path=/home/mario/.arduino15/packages/arduino/tools/openocd/0.11.0-arduino2 -prefs=runtime.tools.openocd-0.11.0-arduino2.path=/home/mario/.arduino15/packages/arduino/tools/openocd/0.11.0-arduino2 -prefs=runtime.tools.arm-none-eabi-gcc.path=/home/mario/.arduino15/packages/arduino/tools/arm-none-eabi-gcc/7-2017q4 -prefs=runtime.tools.arm-none-eabi-gcc-7-2017q4.path=/home/mario/.arduino15/packages/arduino/tools/arm-none-eabi-gcc/7-2017q4 -prefs=runtime.tools.rp2040tools.path=/home/mario/.arduino15/packages/arduino/tools/rp2040tools/1.0.6 -prefs=runtime.tools.rp2040tools-1.0.6.path=/home/mario/.arduino15/packages/arduino/tools/rp2040tools/1.0.6 -prefs=runtime.tools.dfu-util.path=/home/mario/.arduino15/packages/arduino/tools/dfu-util/0.10.0-arduino1 -prefs=runtime.tools.dfu-util-0.10.0-arduino1.path=/home/mario/.arduino15/packages/arduino/tools/dfu-util/0.10.0-arduino1 -verbose /home/mario/Arduino/Carrier_Board_Test/Carrier_Board_Test.ino
Board sfnrf52840pb (platform mbed_nano, package arduino) is unknown
Error compiling for board SparkFun MicroMod nRF52840 Processor Board.
Thank you very very much for any help in advance and have a nice day.