Hello,
it depends on what version of the MbedOS you have in your project.
The Serial API is deprecated and can be used only in MbedOS < 6 for MbedOS =>6 there are new APIs BufferedSerial and UnbufferedSerial . On APIs pages you will also find a basic examples. With these new APIs you also can not use printf
function. But you can still use it for a debugging .
BR, Jan