How to Generate a Random Number, Which is an Element of Zn (Partially Blind Signature Program)

I am trying to implement a partially blind signature scheme in Java from a paper. In equation (1), it requires two random elements r and u to be generated, which must be a subset of Zn.

I am not entirely sure how this may be achieved - do I just need to generate a random number, which is not devisible by p nor q? I assume this could be done using BigInteger gcd, but I am not sure this is actually correct.