Hello, @Tomás Silva !
Why was my quota increase request denied for the HBv3-series VM?
The HBv3-series is only offered in a limited number of Azure regions which limits availability, especially for spot instances (which offers discounts for unused/idle VM hardware). It may be that 120 cores is more than the hardware that is available so you may want to try starting at 16 or 32 cores.
From everything I can see, support was correct when they said that this is "due to very high rates of capacity demand". Another alternative would be to select a more common VM series if you are set on using Azure Spot instances.