The cells that sense sweetness are located on the tip of the tongue. When you lick a candy on the tip of your tongue, it feels particularly sweet because the cells that perceive sweetness are concentrated on the tip of the tongue.
Thus, sugar stimulates the sweet-scented cells on the tongue. This is why we feel sweetness.
Salt does not stimulate the sweet-scented cells even if it touches them. Therefore, we do not feel sweet when we lick salt. Instead, salt stimulates the cells that feel salty.