Can't install twrp or even root. Need help.

I'm having some issues installing/flashing twrp on my Nexus 5. I have been trying to fastboot twrp but it doesn't work, it says it's successful but I don't have twrp. (know what I mean?) I then tried to sideload Superuser so then I can download twrp manager or flashify and install it through either one, but when I do I get an error saying:
E: failed to verify whole-file signature
E: signature verification failed
Installation aborted.

I am out of options.

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