As president, she cannot "lower grocery prices on day one". She can attempt to stimulate the economy (which will take time) with tax cuts (for all, like Trump did) and other things in her control -- to allow producers, suppliers, and stores to cut costs, lowering prices organically.
This "day one" nonsense she spouts has no experience to back it up.
Meanwhile when Trump says "day one" Americans know that he has the experience and proven track record to get the job done and return confidence to our economy.