1. Please check if your QMK Toolbox is the latest version. You can check out the latest version here.
2. Follow this step-by-step guide.
3. If you are a Windows user, please check if your Windows device has the proper driver for the keyboard. Follow the steps below to double check the driver.
1) Open the Windows Device Manager.
2) In Device Manager, locate and double-click on the device you are using.
3) In the device Properties window, click on the Driver tab.
it may appear as a "Guillemot STM DFU Device" rather than an "STMicroelectronics STM32 BOOTLOADER". If the screen shows Guillemot STM DFU Device, that means the computer has the wrong driver installed.