I went into the cyanogen recover menu and did a full factory reset before upgrading to LinageOS. Now when I try to sideload a package to the phone is shows "E: Signature Verification Failed"

Also, when I type

adb devices    

I get back

List of devices attached
1a1e7719        unauthorized

Any help please! There is no OS to boot to so I cannot enable/disable USB Debugging etc.

Also, when I try to adb push I get the following

error: device unauthorized.
This adbd's $ADB_VENDOR_KEYS is not set; try 'adb kill-server' if that seems wrong.
Otherwise check for a confirmation dialog on your device.