We’ve just announced some changes to the Google Maps APIs authentication and usage limits
Usage of the Google Maps APIs now requires a key.
Are you using the Maps API from localhost or a recently launched domain, or was your domain active prior to today? Just wanting to check, since all previously active domains should have been grandfathered for keyless usage, so a previously active domain getting the MissingKeyMapError may be a bug.
In other words, if you weren’t already using Maps API on your domain before June 22nd, 2016, you will be forced to get a API key to get rid of this nasty error.
Keep in mind that each free API key comes with a 25,000 map loads per project per day limit (you can always create multiple projects, though). For very large websites, you could possibly need a Pemium Plan license.
This obviously apply to every enviroment, disregarding the platform (WordPress, Joomla, etc.) or plugin you are using.
Don’t be afraid, getting an API key (and integrating it) is really fast and simple:
1. Follow google developer link and click on Get a key: