Quote from C3RA
GUYS I KNOW THE ANSWER, I'LL COPY AND PASTE FROM THE PIXELMON WEBSITE:
-THIS IS JUST FOR THE SINGLEPLAYER CRASH
What you need to do is go to .minecraft and and delete you database folder. Then redownload pixelmon and drag the database folder into your .minecraft where it was before. Finally before running minecraft go into the config folder in .minecraft go into pixelmon config file using notepad and find the auto update function and set it to false. This made my single player work
It didn't work for me. I would pick Charmander and it wouldn't even appear in my sidebar and when I did PokeSpawn, I didn't get an error message and the pokemon didn't appear.