Since you have a .tgz file, I would assume you downloaded the factory image for your device. You cannot use
adb sideload with those archives. They are not packaged for that. You note in your comment:
The instructions also said to do it as a .zip
...and the instructions are correct. If you find that your environment is deviating from the stated instructions, then it is quite often an indication that you are not following the process the way it is intended. In this situation, luckily, the recovery system bails out without damaging your device, but in the future it would be a good idea to take some care when attempting to install images to your device.
The top of the factory images page you got the update from has different instructions for flashing the contents of the archive. You need to either follow those steps or obtain a copy of the update in a .zip format that is compatible with recovery-based installation.
If you intend to use the factory image, the proper steps are:
Caution: Flashing a new system image deletes all user data. Be certain to first backup any personal data such as photos.
- Download the appropriate system image for your device below, then unzip it to a safe directory.
- Connect your device to your computer over USB.
Start the device in fastboot mode with one of the following methods:
Using the adb tool: With the device powered on, execute:
adb reboot bootloader
Using a key combo: Turn the device off, then turn it on and immediately hold down the relevant key combination for your device.
For example, to put a Nexus 5 ("hammerhead") into fastboot mode, press
and hold Volume Up + Volume Down + Power as the device begins booting
If necessary, unlock the device's bootloader by running:
fastboot oem unlock
- The target device will show you a confirmation screen. (This erases all data on the target device.)
- Open a terminal and navigate to the unzipped system image directory.
- Execute the
flash-all script. This script installs the necessary bootloader, baseband firmware(s), and operating system.
If you are using Windows, you will need third-party software to extract the .tgz archive, because Windows does not support it natively. 7zip is a free (and popular) option.