My teenager has no motivation and no job, what do I do?

The word motivation points in the wrong direction, and that is most of why nothing has worked.

Motivation gets treated as something a kid either has or lacks, so the plan becomes installing some. For almost everybody it runs the other way. You do a thing, something happens, and wanting to do it again shows up afterward. A kid who has never earned money from a stranger has no evidence they can, no evidence means no starting, and no starting means the evidence never arrives. From outside, that closed loop looks exactly like not caring.

Underneath it there is usually one of three specific things, and they need almost opposite responses.

No idea what they would do. Every idea they think of feels too big, too embarrassing, or obviously stupid, so they say nothing. Most common by far, and it is an information problem rather than a character one.

Frightened of the asking. They know exactly what they would do and cannot face saying a price to an adult. This hides well, because looking bored is easier than admitting it.

They do not think it would work. Nobody would pay me. That is a prediction rather than laziness, and it is testable in an afternoon.

What does not help is making the general case. You are arguing that work is good and they do not disagree. They are avoiding one particular conversation with one particular neighbor, and a general argument cannot touch that. Your own story about a job at fourteen lands as a comparison they are losing, whatever you meant by it.

What does help is smaller than you think. Ask what they would want if money were coming in, then be quiet long enough for the second answer. Find the number attached to it. Then make the first step embarrassingly small and countable, like asking five people this week.

One honest exception. If the flatness covers everything rather than just this, if things they used to enjoy have stopped mattering, if it has run for months or sleep and appetite have changed, that is a different situation and a business is not the answer. Talk to a doctor.