I am sorry to hear it didn't work. Mine didn't work either. Had BFP in April but MC in May.
Usually clinics recommend to have at least one full bleed after your cycle, that is, bleed after BFP, next period and then wait for another to start medication.
It does entirely depend what your clinic will recommend based on how your previous cycle went and your health in general.
It might be that they say you can go ahead straight away but if the clinic is busy enough you might end up waiting anyways.
But from my experience, I wanted to go ahead straight away. But you do end up waiting ages for the next period, mine was 56 days. My clinic appointment was on CD6 (busy clinic, I booked app immediately after MC and took what I was given and that was in 2 moths).
It is a good time to calm down. Emotionally and physically. Your ovaries just had a massive party, I think it is better for them and you to have some rest.
I am waiting for my next period, which I think will take ages too. I am on day 10 now.
I was absolutely heart broken and I honestly think it is for the best to wait a bit.