Is there a concept that one can only make the blessing of Asher Yatzar if the toilet is flushed afterwards. Or can one say it even if he did not flush the toilet
Such a concept is not familiar to me. A person that excuses himself in a forest, says Asher Yotzar even thought there is no toilet to flush. It is possible that there is a bit of confusion because there is a halacha that we may not say a bracha, daven or learn near human excrement, but we must distance ourselves 6 feet from it in order to say the bracha. This is not dependent on specifically flushing the toilet. Additionally, we don’t make brachos in a bathroom, even if the toilet has been flushed.