Global functions are not supported in this predicate

Use predicate as a static function

https://developer.apple.com/documentation/swiftdata/filtering-and-sorting-persistent-data#Define-a-filter-using-a-predicate

filter: predicate(),

static func predicate() -> Predicate<Flashcard> { 
  let now = Date()
  return #Predicate<Flashcard> { $0.nextReviewAt <= now } 
}

extract the Date.now to

let date = Date.now

Then, create your predicate using the constant date value. The Date.now is not a constant. This should fix the problem. It did in my test.