This isn't a lack of skill or understanding, it's a lack of attention to detail, pride in workmanship, and effort. This isn't hard stuff, reading requirements and making sure they are satisfied. It doesn't take any technical expertise beyond what is required to understand the requirements. There's no excuse.

You've got to tell this person that it is unacceptable to turn in work that is incomplete or that doesn't adhere to requirements. If they continue to do so, put them on a formal PIP. If they still don't improve, fire them. I have put several people on a PIP and most of them turn it around very quickly once their deficit is laid out in front of them on paper.

I have unsuccessfully tried to mentor two people through this situation and ended up firing both after great expense to the company, their peers, and myself.

Don't underestimate the negative impact this person is having to the rest of the team. You WILL lose good people over time if you allow low performing people to skate, it diminishes the value of good work and causes people to think "why should i continue to bust ass if Larry over here fucks up constantly and we both get paid"?