Unfortunately, this button has been changed from Add to shipping cart to details button.
Can anyone give me solution?
You apparently have added variants to this product so the purchaser has to take a decision first which variant to add to the cart.
Could be.
Also could be the option (switch) „Kaufen-Buttons in Produktlistings anzeigen“ (German translation, gues in English it is called: show buy button in listings) under Settings > Products