VREFBUFF is not available in STM32L431RCTx series, in datasheet it tells that "internal voltage reference buffer is not available", is there any other option to give External ADC reference for this series of STM32.
ADC reference is always what's on the VREF+ pad, which in the STM32L431Rx is internally bonded to VDDA. So, in your case, the ADC reference is whatever voltage is on VDDA (in respect to VSSA).
