I plan to make a small modification to v1.18 to fix the encoder being reversed on my ATS-20+ unit. To check that my build environment is sane, I first tried building the untouched v1.18 source code ...

The first sketch auto-opened by the IDE presently may have corrupted state which causes uploads to fail. To work around this, you can change the board once (to anything) and then change it back to ...