Arduino for STM32

Are you sure you used USART3? (Which pins?)
You could read the Errata Sheet, they are some point around UART, ex: Data corruption due to noisy receive line.

Hi which library you used ?
Issue is that the type used in the library is not correct to call portOutputRegister

OK. so probably issue comes before.
Hard to help more, will see if I can have this board to test on my side but not sure to have one.

Try with it enabled else you don't have the symbol for debugging.

did you build with Optimize for debug selected in the menu ?

No. My guess is that error handler is called during the clock config. You can redefine it at sketch level. https://github.com/stm32duino/Arduino_Core_STM32/blob/8f337e8aa4ce0eee00c75b15102b81e09e1b2b6e/variants/STM32G4xx/G491RC(I-T)_G491RE(I-T-Y)_G4A1RE(I-T-Y)/generic_clock.c#L24 Replace WEAK by ext...

If you pause you should see where you are. If in error handler the the clock config fails.

Maybe system core clock config failed. You should try to debug to see if you reach error_handler.

Got the issue. You select the generic . Select the discovery board menu then the correct part number.

You need to flash the copro binary as stated in the readme.