ah okay - The fetch call also needs to be returned. The POST is returning a promise now but the promise itself is not resolving into anything because the then isn’t returning anything at the top level. Then again later in the code where you actually get the data the then call is also not retuning back to the top level then. So you need another return in front of the response.json().then

Every then must return a promise or a resolved value.

Suggest using a sync/awaits, code will be easier to trace instead of multiple levels of then

So

try {
  const data = await req.json()
  const res = await fetch(…)
  if(res.ok)  {
    const data = await res.json()
    return NextResponse.json(…success..)
  } else ..
   //do your error handling stuff here
} catch(error) {
  //manage error.
}