Robinhood sell option not executing

Is this because no one is buying, or did I place the wrong type of order?

No one here could know what kind of order you placed so there's no possible way to tell you if your order placement was incorrect.

I bought a few CCL calls and I am looking to sell. I placed a "limit sell" at consistently 10-20% under market price (there are large fluctuations), but it is not executing (1 day has passed

I trade CCL options and they are liquid with fairly narrow spreads. This applies the earlier weekly and monthly expirations. The deeper ITM the options are and the further out in time the expiration (say 6 months or more), the wider the B/A spreads get.

Robinhood does some funky things and my guess is that you may be looking at the midpoint (the average of the bid and ask), thinking that is the market price.

To get an accurate answer you'd have to provide more information. What is the strike price and expiration of your call, the respective bid and ask price and what you are trying to sell it for?