I recently ran into the above error after attempting to encrypt my Verizon Samsung SM-G900V (Galaxy S5, klte) running LOS 14.1 (that was a mistake.) Basically it means TWRP can’t successfully decrypt the filesystem.
When this happens, your best bet is to simply factory reset the device and restore a backup. To do this, simply tap Cancel at the decryption password prompt in TWRP, then factory reset via the Wipe menu as usual.
Unfortunately, this will totally negate whatever encryption you had – aside from adopted storage – but it should get return your device to a usable state.