In Pokémon Go, why does one of my Pikachu have an option to evolve, but another one doesn't?
A rather vague question with no images, however given the timing I'll assume it is to do with the recently released Straw Hat Pikachu. This event Pokemon has been specifically set so that you can't evolve it.
Comment on twitter from Niantic Support
Q: why can't we evolve pikachu in this event?
A: Hello Trainer! Straw Hat Pikachu is a special event Pokémon and hence cannot be evolved into Raichu. Thank you for your question! Let us know if we can help you with anything else. ^DS
Niantic Support Twitter