'Serial' was not declared in this scope

[StephiB]

Using 1.8.5 IDE and AVR Boards 1.6.20 tried many different combinations but without success.
Always the same error if Serial is called in setup:
DigitalReadSerial:27: error: 'Serial' was not declared in this scope

Serial.println(buttonState);

^

exit status 1
'Serial' was not declared in this scope

[awatterott]

For the mega328PB you will need the AVR Boards 1.6.200. Further infos here