The flush valve isn't closing, allowing the water to run straight out into the pan. Either the button is jammed, or the mechanism is jammed/broken or the large sealing washer on the bottom of the flush valve plunger is stiff or damaged/defective. You need to investigate all of the flush valve and the flush button linkage in order to find the exact cause, or you could just replace the whole thing with a quality one, they are not expensive. I like fluidmaster fill and flush valves, I fit those for customers I care about - however other quality brands are available. There is also a great deal of cr*p available in the WC valve marketplace. I dislike Wirquin, for example.
Sparks (the Plumber), May 2018
