I do not see at first glance something wrong in your code... Are you sure about your connection (physical connection to connect Tx to RX (Pin 3 CN9 D2 connected to D8 pin 1 CN5) ? If yes, if you put a breakpoint into the RX callback and TX callback, ...

Are you entering in your RX callback (complete callback) setting up a break point in your while (1) inside this callback in your main ?

It looks like the link to the reception in not good. are you well handling the pin affectation for RX and TX into the stm32f4xx_hal_msp.c files as well as the correct alternate function number set ?

Hello,did you check with an oscilloscope that what you got on TX pins you have it on RX Pins on the corresponding GPIO ? Which product is targeted (package as well) and which tutorial are you running ?Best regards,Christophe