First u need to use CFAUTO-ROOT to rootI tried -1 on my encrypted phone, two issues:
1) TWRP doesn't stick. It will revert to stock recovery after 1-2 restarts.
2) Flashing SU doesn't work. Get's an error due to the encryption.
Then u need to install flashify
Then flash the twrp.img using flashify...
Sent from my Nexus 5X using XDA-Developers mobile app