Yes to what Carolita said- natural (non dutch-processed) cocoa can have a more bitter taste than dutched, and this recipe wasn’t designed for it. I would imagine that to change the recipe for a natural cocoa you would need to do something like incorporate baking soda or increase sugar, both of which would require adjustments to other ingredients to make the cake work.
I assume you are making the chocolate velvet, the one Rose pictures with a marzipan candle, not a chocolate version of the red velvet, is that right? If you are making a chocolate version of the red velvet, then there are a number of ingredients, including red dye and natual cocoa, which can produce bitterness. The red velvet is a particularly acidic batter (to preserve red color), and may come across as bitter.