This post relates to the Community Yard Sale Plugin
Google has changed Google Maps so that all websites with domains registered after June 2016 require an API key to work.
If you are using the Community Yard Sale plugin, and you see an error such as the following, then you need an API Key.
You must obtain a Browser API Key from Google. This requires that you have a Google account.
Go to the Google Maps API page, and click the GET A KEY button.
Ensure Create a project is selected and click Continue.
Pick any Name you like and set the URL of your website as shown and click Create.
Copy the key that is created.
Go to your WordPress dashboard, Plugins -> Yard Sale -> Options tab, enter the API key and click Save Changes.