this could be due to file corruption. try to format your file system using this .uf2 file: nRF52_Internal_Format/Meshtastic_nRF52_factory_erase.uf2 at main · markbirss/nRF52_Internal_Format · GitHub
before you transfer the file, make sure you are able to connect a serial monitor to the device, you will need it to send a key press.