Work around found: Fresh/clean install of Arduino Install M5Core2 in Library Manager & opt to include all 15 dependencies (will fail with "no protocol:" error) Install manually the 5 remaining dependencies that failed (MFRC522_I2C,M5_BM8563,M5_ADS1100,M5_ADS1115,HX711 Arduino Library,PCA9554) Install M5Core2 v0.0.1 in Library Manager (downgrade) Update M5Core2 in Library Manager to latest version (0.0.6), will list 0 dependencies and ask if you would like to install them, but select "Install 'M5Core2' only". Can now compile and run helloworld example (and others too). Confirmed same install problems encountered on a second PC, and same work around solved it. Arrgh~! Shouldn't be this messy/difficult.