Serial.println does not work in added tabs because of error on your side where you declare Serial0
Hi @arduin24. Thanks for your report. I just gave it a try and it works fine for me:
Please make a sketch that demonstrates the problem, then share the sketch by following this tutorial:
https://docs.arduino.cc/arduino-cloud/cloud-editor/share-your-sketches/
Post the URL of the shared sketch in a reply here on the forum and we'll investigate further.
What is Serial0 ? On what board do you use it?