How do you reject a job offer gracefully?

I can't possibly be qualified in the technical role and the company knows this full well(The role I applied for isn't available any longer). They still offered me another route to an engineering job anyway. I get the feeling they liked my interest in business related things despite being a dev (after a long non-technical interview we all enjoyed), but I know their stack is not one of my competencies. Been fumbling mess over this for at least a week.