In this video

Remove the lid from the toilet tank and visually inspect the components. Check the chain that connects the flush handle lever to the flapper. If the chain is too tight, it can prevent the flapper from sealing. If it's too loose, it may get caught under the flapper. Also, inspect the flapper itself. Lift it and check for any signs of warping, brittleness, or damage that would prevent it from creating a watertight seal over the flush valve opening.