If I had to pick one I would say resilience, since you need to keep trying lots of different things until you find product market fit, and even when you do you still need to keep evolving. Lots of experiments will simply fail.
@abadesi Of course, the first indicator of effectiveness is the profitability of the business. If you have buyers who bring money, this is a success. But remember that it is very important not to stop there and continue to develop your business, not forgetting about advertising. I can share with you an article https://www.techwalls.com/build-... where they write about how to create effective branding of your product on social networks.
I think having the flexibility to adapt to any situation or adjust your work according to unplanned circumstances is pretty essential. You never know what issues or roadblocks you're going to run into and you need to be able to run with any of them