i have a problem using timer 1 fast pwm prescaler 64 tinkercad

[DrDiettrich]

@blh64

    if ( step >= 10 ) {
      step = 1;
      OCR1B = 250;
    }
    TCNT1 = 0;   

Updating TCNT1 in the ISR is not required, perhaps calling for trouble. See the data sheet what the FastPWM mode does in hardware with the timer registers and output pins.

I'd use a variable or constant for TOP:

const uint16_t T1TOP = 2500;
...
  OCR1A = T1TOP;
...
  OCR1B += 250;
  if (OCR1B>=T1TOP) ...