Need advice, company is denying me a retest after a single negative dilute drug test despite company policy stating a one time retest is allowed.

This is mostly a vent over anything else, but I would greatly appreciate some advice on what to do now.

After 2 weeks of paid training through a staffing agency and a successful completion of a pre hire test, I received an offer letter and I'm supposed to start on May 4th.

My background check came though completely clear but my urine drug test came back as a negative dilute and I received a pre-adverse action notification because of it. The company's policy states that candidates would be subject to a one time retest within 24 hours upon notification of such a result.

After contacting HR, I wasn't given any instruction on retesting and was informed policy would not allow for a retest in this situation.

I'm just not really sure where to go form here. Currently waiting for replies from two members in HR, and my start date is before the end 5 business day window given in the pre-adverse action notification.

Is there anything else I can/should do? I should still go in for that first day right? I just can't believe I'm going to lose this job over accidentally drinking too much water.