Arduino L led glows continuously
Don't let that pin float undefined: If undefined (INPUT), pull it to GND to have the led OFF.

If ever possible, try to use the LED_BUILTIN pin as an OUTPUT pin.

And signal some information there. On an UNO (or other 328-based Arduino), this hint might collide with the SPI_SCK pin, which is D13 (PB5 on an atmega328). But as an SPI Master, this pin is at least OUTPUT (and thus will signal SPI activity).