I uploaded the exact same Nina W10 code with GPIO_NUM_21 and platformio.ini to my ESP32 development board, and it blinks IO-21 without a problem.
(Not suprising since the Nina W10 is also just a ESP32 target with the same code running on it, so a change in development board doesn’t make difference here…)
So I strongly sense you have a hardware issue. Please check if you see a 3.3V / 0V switching on IO-21 on J2 using a multimeter (or a LED with resistor). Lower the frequency if necessary.
