toilet is constantly running water to tank but won’t fill
Get some food coloring and put a lot of it into the tank, you will likely see it ending up in the bowl. When it leaks into the bowl slowly, it just goes down the drain slowly without creating a "flush" action.

If you start putting the food coloring in next to the overflow tube in the tank, you will see it go into the bowl immediately, which means your fill valve needs adjusting so that it does not let the water go over the top of that tube.

If it is not, then when the coloring filters its way down to the flapper, that's when you will likely start to see it make it's way into the bowl. that means you must replace the flapper.