[other] Decrypt LUKS lvg from USB stick [ubuntu server 14.04]

Hi everyone, this is my first thread in the comunity.
I've recently encrypted my ubuntu server 14.04 (installed on a RAID 1 in a lvg) so I encrypted only the / partition with a specific passphrase, swap has a random passphrase.
Now what I want to do is to be able to decrypt or auto fill the passphrase from an usb stick, I mean, having in a file a passphrase which is like this (not the same) /aFgS@nTb&$ (0HR!F_&z6uKe@!{~j2+] but its long and complex to write as this one (I want to use the same I've defined)

All the tutorials suggested creating a new one in a file, but I have this error when adding that new one to the LUKS keys.
Im attaching a screenshot of the last things I did.
This are the tutorials I've tried
https://www.centos.org/forums/viewtopic.php?t=53452
https://www.howtoforge.com/automatic...with-a-keyfile