Why does North Korea say “no talks with South Korea”?

Because the US and UN sanctions have effectively hamstrung South Korean economic cooperative initiatives with North Korea while the US assumed a hardline maximum pressure posture after the US- North Korean summit in Singapore. The Moon government now finds itself characterized by North Korea as the proverbial quisling state under the boot of the foreign occupier. Without ability to demonstrate the freedom of action to materially cooperate with North Korea beyond miltary initiatives around the dmz and maritime northern limit lines and some cultural and sports interactions the South appears incapable of escaping the limits imposed by US hardliners. When South Korea set up a liaison office in Kaesong North Korea simply for diplomatic purposes critics accused the Moon administration of violating sanctions. US officials said South Korea was getting too far out in front of the US and that there should be “no daylight” between Seoul and Washington with respect to North Korea. The US position is basically no sanctions relief until final full verifiable denuclearization. The US administration does not agree with a step by step process of action for action to build trust on the basis of reciprocity. Some have criticized the US approach as an unrealistic “all or nothing” approach causing the negotiations to deadlock such as occured at the Hanoi summit. The practical effect of the US negotiating posture has severely restricted South Korea's freedom to offer incentives to North Korea to engage in further dialogue with itself or the US.