It appears the latest Sparkfun Arduino Boards doesn’t have the Atto84 support.
<LINK_TEXT text="https://raw.githubusercontent.com/spark … ndex.json
">https://raw.githubusercontent.com/sparkfun/Arduino_Boards/master/IDE_Board_Manager/package_sparkfun_index.json
</LINK_TEXT>
The old Sparkfun Arduino Boards doesn’t work and conflicts with the latest v1.8.9 IDE.
<LINK_TEXT text="https://raw.githubusercontent.com/spark … ndex.json
">https://raw.githubusercontent.com/sparkfun/Arduino_Boards/tiny/IDE_Board_Manager/package_sparkfun_index.json
</LINK_TEXT>