Shield update failed - getting error

If you're updating Shield to the newest version and it fails (you get error), that means that something has interrupted upgrading process. 

The errors appear to indicate that the Shield plugin installation during the upgrade broke or was interrupted

It is the responsibility of WordPress itself (and the webhost) to perform and complete a plugin installation or upgrade, and there's no way we can intercede to address this issue.

Some causes for failed installation and upgrade:

  • Not enough diskspace - check using your hosting control panel that you have enough diskspace
  • Incorrect filesystem permissions - check with your control panel, or with your hosting support, that the filesystem permissions for the Shield plugin directory (/wp-content/plugins/wp-simple-firewall/) are valid and sufficient to allow upgrades through the WordPress dashboard.

One approach is to completely remove the plugin and then reinstall it from WordPress.org. The plugin .zip file can be downloaded directly from here:
https://wordpress.org/plugins/wp-simple-firewall/

The good thing about WordPress now is that when it does hit this sort of problem, it doesn't completely break the site allowing you to repair such things. You can safely delete the plugin and re-install it.