What is the Working Temperature for TSL 2591? Is it Okay to put it on Direct Sunlight? Will it still work?
TSL2591 Datasheet (adafruit.com) page 5:
TA Operating free-air temperature -30 to 70 ºC
Your very best source of information about the TSL2591 is the data sheet, and the allowed temperature range will be stated in it.
Also note that the datasheet specifies the device as a 3.3V variety so don't power it up with a 5V arduino.