ReLU derivative in backpropagation

Relu Derivative is 1 for x >= 0 and 0 for x < 0

The relu derivative can be implemented with np.heaviside step function e.g. np.heaviside(x, 1). The second parameter defines the return value when x = 0, so a 1 means 1 when x = 0.

since ReLU doesn't have a derivative.

No, ReLU has derivative. I assumed you are using ReLU function f(x)=max(0,x). It means if x<=0 then f(x)=0, else f(x)=x. In the first case, when x<0 so the derivative of f(x) with respect to x gives result f'(x)=0. In the second case, it's clear to compute f'(x)=1.