What ingbrzy said, but try this.you can not boot TWRP.. you have to flash it.. fastboot flash recovery twrp.img
1. Put phone in the fastboot mode (switch it off, then press and hold power + volume down for a couple of seconds)
2. Flash twrp through fastboot (fastboot flash recovery twrp.img)
3. Switch phone off completly by holding power + volume down again
4. Boot phone into recovery by holding power + volume up
5. The phone will boot into twrp
This is what I did and it installed fine.