Are you sure it's not required, I don't think you can flash twrp without an unlocked bootloader, did you do it without an unlocked bootloader?
Yes, I am shure, I did it 3 weeks ago this way with a brandnew ZE550KL. My bootloader is still locked a...
Need unlocked bootloader for flashing this rom? Am currently in stock Asus rom.. please help
Unlocking is not required.
1. Update your phone to stock MM
2. Boot into fastboot
3. flash twrp via fastboot "fastboot flash recovery twrp....img"...
This one works for my encrypted phone (encrypted with CM14.1):
Even with encrypted f2fs (changed to f2fs with this one).