I've not seen the show, but I used to play the video games. In Pokemon Go there is a secret way to get Pikachu by walking away from the starting Pokemon. People on the internet say Ash did so in the anime. Is that true? If not, how can this be explained with anime's events?
In the anime Ash overslept and was to late to get Bulbasaur, Charmander or Squirtle. Professor Oak then gives him Pikachu.
It happens in the first episode which you can watch here: https://www.youtube.com/watch?v=_CvBNRxpRqU