As of the 22nd of June 2016 Google maps will no longer be supporting keyless installs.
Any sites that displayed a Google map prior to this date will be offered keyless install support however if you are adding a Google map to your site for the first time after 22/06/2016 then you will require an API key.
Obtaining an API key is a fairly quick and simple process.
Step 1a. Click 'Get a Key'
Step 2. Agree to Googles terms & conditions
Step 3. Enter a name for your API Key (eg: my map) and click 'create'
Step 4. Your API key will have been created.
You will need to copy / paste this key into the Styled Maps add-on whenever adding a Styled Maps block.