I believe your relationships are looking for the columns reviewable_id and reviewable_type.
So to get it to work, you need to change the Photo.php relationship so it can find the columns:
public function review()
{
return $this->morphOne(App\Models\Reviewable::class, 'review');
}
Then you can call $photo->review.
There seems to be some confusion about the terms review and reviewable.
To me it looks like a photo is one model type that can have a Review, so the model should have been named Review, rather than Reviewable.
A photo is then a reviewable model, i.e. it is a model that can have a review. This would then make sense that the columns should be reviewable_id and reviewable_type.