Some nouns like "courage" are quantifiable but not really measurable; one could describe Fred as having "more" courage than Joe, but there's no standard unit of courage that would make it possible to say that e.g. Fred has 2.7 more couragions than Joe.
The lack of defined units, however, doesn't affect "zero" because zero doesn't require units. If something is has a length of zero inches, zero centimeters, and zero angstroms, or zero miles, it has zero length; the units don't matter. If something has a mass of zero ounces, zero milligrams, or zero tons, it has zero mass; again, the units don't matter. That principle can be extended to things like "courage": it's not possible to specify any particular non-zero quantity of courage because that would require defining units, but since "zero" doesn't require units, it is possible to have zero courage.