First thing’s first, enter the Pokemon Center and heal your Pokemon. Once you’re done, go back outside and continue right. You’ll once again talk to Alder before he walks off to your north. Enter the first house to the right of the Pokemon Center and talk to the bald man inside. He’ll give you a Potion .
Exit and move north up the street. There’s not really anything else of note to do here, so continue north until you reach Alder once more. You’ll talk for a third time. Once you have, he’ll stand guard over his house. He won’t let you enter until you’ve completed Route 20, which includes Floccesy Ranch.
So instead follow the path past his house and enter Route 20.