It looks like FastLed doesn't work on Photon2. Just including the library on a barebones project breaks:
In file included from lib/FastLED/src/FastLED.h:34,
from src/fastled_stuff.cpp:2:
lib/FastLED/src/led_sysdefs.h:19:10: fatal error: platforms/avr/led_sysdefs_avr.h: No such file or directory
19 | #include "platforms/avr/led_sysdefs_avr.h"
Selecting Photon as the target causes no such problem.
FastLED does not work on the Photon 2, P2, or M-SoM. It's unlikely to be fixed as it would require a major rewrite to the library to be able to work. See libraries in the migration guide.