Owners of Google Pixel devices on Android Pie most likely have a PIN / password setup to secure your device, and if you want to create a Nandroid backup (or flash a file from the internal storage), you would typically need to remove the protection method before booting into TWRP recovery, to ensure that encryption is disabled.
However, the lead maintainer of TWRP has announced decryption support for the Pixel and Pixel 2 phones – TWRP version 3.2.3-1 is now able to decrypt the data partition after entering the PIN / password on your device – this new version is only available for the Google Pixel, Pixel XL, Pixel 2, and Pixel 2 XL.
TWRP is hands down the most popular custom recovery, because it supports a majority of devices and offers full backup and restoring, touch support, A/B slot selection, easily flashing .zips, etc. There are similar custom recoveries (CarlivTouch, etc) but TWRP is the preferred recovery for most modders, using the others as a fall-back if there is no TWRP port for a particular device that needs custom recovery – or they may just try to flash things via ADB.
TWRP is constantly maintained and ported to devices, so of course there is always little bugs to be ironed out between versions – for example, data decryption not working on the Pixel devices mentioned in this article. Fortunately, its one issue we can count as eradicated, so Google Pixel owners should definitely head to the TWRP website to download this latest version of the custom recovery.