Kung Fu Panda (2008) was delisted from the PlayStation Store when the PSP/PS Vita storefronts shut down in July 2021 (web) and October 2021 (on-device). You can no longer purchase or download it digitally.
If you have a PSP-1000/2000/3000, hunt for a used UMD. Otherwise, play on PS2, PC, or Xbox 360.