Can’t evolve one of my Pikachu in Pokemon Go
This happened because you caught a glitch! A better way of putting it is, if you close down your game and restart it, you'll see this Pikachu is supposed to be wearing a costume. It's a fall event Pikachu, and the costumed Pokemon caught during this event can't be evolved. Since you caught this when your phone didn't have the assets (images etc.) to show the costume, it looked like a regular Pikachu, which should have been evolvable, hence the confusion.