How to wire programmable switch to only a single set of white-black-ground wires
You'll need to find something that doesn't need a neutral, or run a /3 cable to replace the switch loop

Unfortunately, the timer switch you have needs a neutral, which your old-style switch loop wiring does not provide.  So, you'll either need to find a different switch that doesn't need a neutral, or to replace the existing cable with a /3 cable so that neutral can be provided at the switch location in addition to line and load.