This is such a tough question to answer because there are so many important qualities and skills that need to go hand-in-hand to be an entrepreneur.
If I was to pick only one quality, I would pick "Grit". To me, grit is the perseverance and resilience to relentlessly pursue a goal. To have grit means that you're going to continuously learn from your successes and most importantly, your failures as well and keep on continuously improving to achieve your goal(s). Without grit, it may be easy to fall into discouragement or lack of drive to learn and put in the work to succeed as an entrepreneur.
Hopefully that makes sense!