Hello Claude,
Hope you are doing well.
I'd recommend to grab the hello world example and using the BOARD_InitDebugConsole(); function as reference to initialize what you'd like to work with. Once you have it working in this example you may merge them together with your custom application.This function uses the SerialManager_OpenReadHandle and you may see how it is used.In addition if you would like to understand the API here is the link for more details on the functions and returned status.
MCUXpresso SDK API Reference Manual: Serial Manager
Also check what is returned. The reading handle can not be opened multiple at the same time. The error code kStatus_SerialManager_Busy would be returned when the previous reading handle is not closed. And there can only be one buffer for receiving for the reading handle at the same time.
Best Regards,
Sabina
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