[Q] Can't flash any custom recovery on my m8 topic

My m8 has an unlocked bootloader with s-on and i recently just used a nandroid backup which flashed the stock recovery and i updated to newest 5.0 update my phone was with t-mobile but it is factory unlocked now. Whenever I try to flash philz or twrp through fastboot it gives me:

target reported max download size of 1826414592 bytes
sending 'twrp' (16086 KB)...
OKAY [ 1.469s]
writing 'twrp'...
(bootloader) signature checking...
FAILED (remote: signature verify fail)
finished. total time: 2.047s

How am i supposed to flash a custom recovery